I have been this kind of errors since a month ago, and it's going worse. At first it affected MP3 playing, after that, video playing. Now, Synaptic is affected too and it looks as if my HD is getting rotted...
I own a HP Pavilion tx2000, installed Ubuntu 10.04, intensive user for engineering: python programmer, cross-compiling kernels form ARM platforms, Arduino programmer, PCB designer...
I have Windows 7 as main (and seldom used) OS.

I did fsck but it says my partitions are in PERFECT state and shape.

My fstab:

# /etc/fstab: static file system information.
#
# Use 'blkid -o value -s UUID' to print the universally unique identifier
# for a device; this may be used with UUID= as a more robust way to name
# devices that works even if disks are added and removed. See fstab(5).
#
# <file system> <mount point>   <type>  <options>       <dump>  <pass>
proc            /proc           proc    nodev,noexec,nosuid 0       0
# / was on /dev/sda5 during installation
UUID=6e3ce0ed-5bf2-4d40-8901-4948064fa010 /               ext4    errors=remount-ro 0       1
# swap was on /dev/sda6 during installation
UUID=692433c3-c222-436a-afb1-76ff99b7500f none            swap    sw              0       0
# Add windows partition and binds
UUID=361EE6161EE5CF45 /mnt/windows ntfs rw,auto,users,exec,nls=utf8,umask=003,gid=46,uid=1000    0   2
/mnt/windows/Documents\040and\040Settings/Ricard/Documents  /home/ricard/windows/Documentos none   bind
/mnt/windows/Documents\040and\040Settings/Ricard/Desktop    /home/ricard/windows/Escritorio none   bind
/mnt/windows/Documents\040and\040Settings/Ricard/Pictures   /home/ricard/windows/Imagenes   none   bind


blkid says...

ricard@ricard-HPtx2500:/var/log$ sudo blkid
/dev/sda1: LABEL="Reservado para el sistema" UUID="3418CD8B18CD4C94" TYPE="ntfs" 
/dev/sda2: UUID="361EE6161EE5CF45" TYPE="ntfs" 
/dev/sda5: UUID="6e3ce0ed-5bf2-4d40-8901-4948064fa010" TYPE="ext4" 
/dev/sda6: UUID="692433c3-c222-436a-afb1-76ff99b7500f" TYPE="swap" 
/dev/sdb1: SEC_TYPE="msdos" LABEL="FAT" UUID="F9F4-0660" TYPE="vfat" 
/dev/sdb2: LABEL="ext3" UUID="4b696001-2cc1-4ba9-921d-8ad6d43d5397" TYPE="ext3" 
/dev/sdc1: LABEL="USB DISK" UUID="44E3-F405" TYPE="vfat" 


Last 50 lines of my /etc/kern.log

ricard@ricard-HPtx2500:/var/log$ tail -n50 kern.log
Sep 16 11:21:02 ricard-HPtx2500 kernel: [15743.241297]          res 41/40:00:ed:a9:72/00:00:1e:00:00/40 Emask 0x409 (media error) <F>
Sep 16 11:21:02 ricard-HPtx2500 kernel: [15743.241311] ata3.00: status: { DRDY ERR }
Sep 16 11:21:02 ricard-HPtx2500 kernel: [15743.241321] ata3.00: error: { UNC }
Sep 16 11:21:02 ricard-HPtx2500 kernel: [15743.251528] ata3.00: configured for UDMA/100
Sep 16 11:21:02 ricard-HPtx2500 kernel: [15743.251564] ata3: EH complete
Sep 16 11:21:04 ricard-HPtx2500 kernel: [15745.733540] ata3.00: exception Emask 0x0 SAct 0x1 SErr 0x0 action 0x0
Sep 16 11:21:04 ricard-HPtx2500 kernel: [15745.733558] ata3.00: irq_stat 0x40000008
Sep 16 11:21:04 ricard-HPtx2500 kernel: [15745.733574] ata3.00: failed command: READ FPDMA QUEUED
Sep 16 11:21:04 ricard-HPtx2500 kernel: [15745.733603] ata3.00: cmd 60/08:00:e8:a9:72/00:00:1e:00:00/40 tag 0 ncq 4096 in
Sep 16 11:21:04 ricard-HPtx2500 kernel: [15745.733610]          res 41/40:00:ed:a9:72/00:00:1e:00:00/40 Emask 0x409 (media error) <F>
Sep 16 11:21:04 ricard-HPtx2500 kernel: [15745.733623] ata3.00: status: { DRDY ERR }
Sep 16 11:21:04 ricard-HPtx2500 kernel: [15745.733633] ata3.00: error: { UNC }
Sep 16 11:21:04 ricard-HPtx2500 kernel: [15745.743883] ata3.00: configured for UDMA/100
Sep 16 11:21:04 ricard-HPtx2500 kernel: [15745.743923] ata3: EH complete
Sep 16 11:21:07 ricard-HPtx2500 kernel: [15748.225803] ata3.00: exception Emask 0x0 SAct 0x1 SErr 0x0 action 0x0
Sep 16 11:21:07 ricard-HPtx2500 kernel: [15748.225822] ata3.00: irq_stat 0x40000008
Sep 16 11:21:07 ricard-HPtx2500 kernel: [15748.225838] ata3.00: failed command: READ FPDMA QUEUED
Sep 16 11:21:07 ricard-HPtx2500 kernel: [15748.225867] ata3.00: cmd 60/08:00:e8:a9:72/00:00:1e:00:00/40 tag 0 ncq 4096 in
Sep 16 11:21:07 ricard-HPtx2500 kernel: [15748.225873]          res 41/40:00:ed:a9:72/00:00:1e:00:00/40 Emask 0x409 (media error) <F>
Sep 16 11:21:07 ricard-HPtx2500 kernel: [15748.225887] ata3.00: status: { DRDY ERR }
Sep 16 11:21:07 ricard-HPtx2500 kernel: [15748.225896] ata3.00: error: { UNC }
Sep 16 11:21:07 ricard-HPtx2500 kernel: [15748.236088] ata3.00: configured for UDMA/100
Sep 16 11:21:07 ricard-HPtx2500 kernel: [15748.236122] ata3: EH complete
Sep 16 11:21:09 ricard-HPtx2500 kernel: [15750.707122] ata3.00: exception Emask 0x0 SAct 0x1 SErr 0x0 action 0x0
Sep 16 11:21:09 ricard-HPtx2500 kernel: [15750.707141] ata3.00: irq_stat 0x40000008
Sep 16 11:21:09 ricard-HPtx2500 kernel: [15750.707157] ata3.00: failed command: READ FPDMA QUEUED
Sep 16 11:21:09 ricard-HPtx2500 kernel: [15750.707186] ata3.00: cmd 60/08:00:e8:a9:72/00:00:1e:00:00/40 tag 0 ncq 4096 in
Sep 16 11:21:09 ricard-HPtx2500 kernel: [15750.707191]          res 41/40:00:ed:a9:72/00:00:1e:00:00/40 Emask 0x409 (media error) <F>
Sep 16 11:21:09 ricard-HPtx2500 kernel: [15750.707205] ata3.00: status: { DRDY ERR }
Sep 16 11:21:09 ricard-HPtx2500 kernel: [15750.707215] ata3.00: error: { UNC }
Sep 16 11:21:09 ricard-HPtx2500 kernel: [15750.717485] ata3.00: configured for UDMA/100
Sep 16 11:21:09 ricard-HPtx2500 kernel: [15750.717520] ata3: EH complete
Sep 16 11:21:11 ricard-HPtx2500 kernel: [15753.199452] ata3.00: exception Emask 0x0 SAct 0x1 SErr 0x0 action 0x0
Sep 16 11:21:11 ricard-HPtx2500 kernel: [15753.199470] ata3.00: irq_stat 0x40000008
Sep 16 11:21:11 ricard-HPtx2500 kernel: [15753.199485] ata3.00: failed command: READ FPDMA QUEUED
Sep 16 11:21:11 ricard-HPtx2500 kernel: [15753.199513] ata3.00: cmd 60/08:00:e8:a9:72/00:00:1e:00:00/40 tag 0 ncq 4096 in
Sep 16 11:21:11 ricard-HPtx2500 kernel: [15753.199518]          res 41/40:00:ed:a9:72/00:00:1e:00:00/40 Emask 0x409 (media error) <F>
Sep 16 11:21:11 ricard-HPtx2500 kernel: [15753.199532] ata3.00: status: { DRDY ERR }
Sep 16 11:21:11 ricard-HPtx2500 kernel: [15753.199542] ata3.00: error: { UNC }
Sep 16 11:21:11 ricard-HPtx2500 kernel: [15753.209861] ata3.00: configured for UDMA/100
Sep 16 11:21:11 ricard-HPtx2500 kernel: [15753.209916] sd 2:0:0:0: [sda] Unhandled sense code
Sep 16 11:21:11 ricard-HPtx2500 kernel: [15753.209926] sd 2:0:0:0: [sda]  Result: hostbyte=DID_OK driverbyte=DRIVER_SENSE
Sep 16 11:21:11 ricard-HPtx2500 kernel: [15753.209942] sd 2:0:0:0: [sda]  Sense Key : Medium Error [current] [descriptor]
Sep 16 11:21:11 ricard-HPtx2500 kernel: [15753.209962] Descriptor sense data with sense descriptors (in hex):
Sep 16 11:21:11 ricard-HPtx2500 kernel: [15753.209970]         72 03 11 04 00 00 00 0c 00 0a 80 00 00 00 00 00 
Sep 16 11:21:11 ricard-HPtx2500 kernel: [15753.210103]         1e 72 a9 ed 
Sep 16 11:21:11 ricard-HPtx2500 kernel: [15753.210118] sd 2:0:0:0: [sda]  Add. Sense: Unrecovered read error - auto reallocate failed
Sep 16 11:21:11 ricard-HPtx2500 kernel: [15753.210138] sd 2:0:0:0: [sda] CDB: Read(10): 28 00 1e 72 a9 e8 00 00 08 00
Sep 16 11:21:11 ricard-HPtx2500 kernel: [15753.210168] end_request: I/O error, dev sda, sector 510831085
Sep 16 11:21:11 ricard-HPtx2500 kernel: [15753.210238] ata3: EH complete

Syslog doesn't add anything...

Sep 16 11:21:02 ricard-HPtx2500 kernel: [15743.241321] ata3.00: error: { UNC }
Sep 16 11:21:02 ricard-HPtx2500 kernel: [15743.251528] ata3.00: configured for UDMA/100
Sep 16 11:21:02 ricard-HPtx2500 kernel: [15743.251564] ata3: EH complete
Sep 16 11:21:04 ricard-HPtx2500 kernel: [15745.733540] ata3.00: exception Emask 0x0 SAct 0x1 SErr 0x0 action 0x0
Sep 16 11:21:04 ricard-HPtx2500 kernel: [15745.733558] ata3.00: irq_stat 0x40000008
Sep 16 11:21:04 ricard-HPtx2500 kernel: [15745.733574] ata3.00: failed command: READ FPDMA QUEUED
Sep 16 11:21:04 ricard-HPtx2500 kernel: [15745.733603] ata3.00: cmd 60/08:00:e8:a9:72/00:00:1e:00:00/40 tag 0 ncq 4096 in
Sep 16 11:21:04 ricard-HPtx2500 kernel: [15745.733610]          res 41/40:00:ed:a9:72/00:00:1e:00:00/40 Emask 0x409 (media error) <F>
Sep 16 11:21:04 ricard-HPtx2500 kernel: [15745.733623] ata3.00: status: { DRDY ERR }
Sep 16 11:21:04 ricard-HPtx2500 kernel: [15745.733633] ata3.00: error: { UNC }
Sep 16 11:21:04 ricard-HPtx2500 kernel: [15745.743883] ata3.00: configured for UDMA/100
Sep 16 11:21:04 ricard-HPtx2500 kernel: [15745.743923] ata3: EH complete
Sep 16 11:21:07 ricard-HPtx2500 kernel: [15748.225803] ata3.00: exception Emask 0x0 SAct 0x1 SErr 0x0 action 0x0
Sep 16 11:21:07 ricard-HPtx2500 kernel: [15748.225822] ata3.00: irq_stat 0x40000008
Sep 16 11:21:07 ricard-HPtx2500 kernel: [15748.225838] ata3.00: failed command: READ FPDMA QUEUED
Sep 16 11:21:07 ricard-HPtx2500 kernel: [15748.225867] ata3.00: cmd 60/08:00:e8:a9:72/00:00:1e:00:00/40 tag 0 ncq 4096 in
Sep 16 11:21:07 ricard-HPtx2500 kernel: [15748.225873]          res 41/40:00:ed:a9:72/00:00:1e:00:00/40 Emask 0x409 (media error) <F>
Sep 16 11:21:07 ricard-HPtx2500 kernel: [15748.225887] ata3.00: status: { DRDY ERR }
Sep 16 11:21:07 ricard-HPtx2500 kernel: [15748.225896] ata3.00: error: { UNC }
Sep 16 11:21:07 ricard-HPtx2500 kernel: [15748.236088] ata3.00: configured for UDMA/100
Sep 16 11:21:07 ricard-HPtx2500 kernel: [15748.236122] ata3: EH complete
Sep 16 11:21:09 ricard-HPtx2500 kernel: [15750.707122] ata3.00: exception Emask 0x0 SAct 0x1 SErr 0x0 action 0x0
Sep 16 11:21:09 ricard-HPtx2500 kernel: [15750.707141] ata3.00: irq_stat 0x40000008
Sep 16 11:21:09 ricard-HPtx2500 kernel: [15750.707157] ata3.00: failed command: READ FPDMA QUEUED
Sep 16 11:21:09 ricard-HPtx2500 kernel: [15750.707186] ata3.00: cmd 60/08:00:e8:a9:72/00:00:1e:00:00/40 tag 0 ncq 4096 in
Sep 16 11:21:09 ricard-HPtx2500 kernel: [15750.707191]          res 41/40:00:ed:a9:72/00:00:1e:00:00/40 Emask 0x409 (media error) <F>
Sep 16 11:21:09 ricard-HPtx2500 kernel: [15750.707205] ata3.00: status: { DRDY ERR }
Sep 16 11:21:09 ricard-HPtx2500 kernel: [15750.707215] ata3.00: error: { UNC }
Sep 16 11:21:09 ricard-HPtx2500 kernel: [15750.717485] ata3.00: configured for UDMA/100
Sep 16 11:21:09 ricard-HPtx2500 kernel: [15750.717520] ata3: EH complete
Sep 16 11:21:11 ricard-HPtx2500 kernel: [15753.199452] ata3.00: exception Emask 0x0 SAct 0x1 SErr 0x0 action 0x0
Sep 16 11:21:11 ricard-HPtx2500 kernel: [15753.199470] ata3.00: irq_stat 0x40000008
Sep 16 11:21:11 ricard-HPtx2500 kernel: [15753.199485] ata3.00: failed command: READ FPDMA QUEUED
Sep 16 11:21:11 ricard-HPtx2500 kernel: [15753.199513] ata3.00: cmd 60/08:00:e8:a9:72/00:00:1e:00:00/40 tag 0 ncq 4096 in
Sep 16 11:21:11 ricard-HPtx2500 kernel: [15753.199518]          res 41/40:00:ed:a9:72/00:00:1e:00:00/40 Emask 0x409 (media error) <F>
Sep 16 11:21:11 ricard-HPtx2500 kernel: [15753.199532] ata3.00: status: { DRDY ERR }
Sep 16 11:21:11 ricard-HPtx2500 kernel: [15753.199542] ata3.00: error: { UNC }
Sep 16 11:21:11 ricard-HPtx2500 kernel: [15753.209861] ata3.00: configured for UDMA/100
Sep 16 11:21:11 ricard-HPtx2500 kernel: [15753.209916] sd 2:0:0:0: [sda] Unhandled sense code
Sep 16 11:21:11 ricard-HPtx2500 kernel: [15753.209926] sd 2:0:0:0: [sda]  Result: hostbyte=DID_OK driverbyte=DRIVER_SENSE
Sep 16 11:21:11 ricard-HPtx2500 kernel: [15753.209942] sd 2:0:0:0: [sda]  Sense Key : Medium Error [current] [descriptor]
Sep 16 11:21:11 ricard-HPtx2500 kernel: [15753.209962] Descriptor sense data with sense descriptors (in hex):
Sep 16 11:21:11 ricard-HPtx2500 kernel: [15753.209970]         72 03 11 04 00 00 00 0c 00 0a 80 00 00 00 00 00 
Sep 16 11:21:11 ricard-HPtx2500 kernel: [15753.210103]         1e 72 a9 ed 
Sep 16 11:21:11 ricard-HPtx2500 kernel: [15753.210118] sd 2:0:0:0: [sda]  Add. Sense: Unrecovered read error - auto reallocate failed
Sep 16 11:21:11 ricard-HPtx2500 kernel: [15753.210138] sd 2:0:0:0: [sda] CDB: Read(10): 28 00 1e 72 a9 e8 00 00 08 00
Sep 16 11:21:11 ricard-HPtx2500 kernel: [15753.210168] end_request: I/O error, dev sda, sector 510831085
Sep 16 11:21:11 ricard-HPtx2500 kernel: [15753.210238] ata3: EH complete
Sep 16 11:23:42 ricard-HPtx2500 AptDaemon: INFO: Quitting due to inactivity
Sep 16 11:23:42 ricard-HPtx2500 AptDaemon: INFO: Shutdown was requested