XP not detected by GRUB if Windows7 installed after XP

[YannUbuntu]

Originally reported here: http://ubuntuforums.org/showthread.php?t=2046134

User installed XP then Win7. os-prober detects Win7 but not XP.

For information:

sdb1: __________________________________________________________________________

    File system: ntfs
    Boot sector type: Windows Vista/7: NTFS
    Boot sector info: No errors found in the Boot Parameter Block.
    Operating System: Windows XP
    Boot files: /boot.ini /bootmgr /Boot/BCD /ntldr /NTDETECT.COM

sdb2: __________________________________________________________________________

    File system: ntfs
    Boot sector type: Windows XP: NTFS
    Boot sector info: No errors found in the Boot Parameter Block.
    Operating System: Windows 7
    Boot files: /Windows/System32/winload.exe

### BEGIN /etc/grub.d/30_os-prober ###
menuentry "Windows 7 (loader) (on /dev/sdb1)" --class windows --class os {
 insmod part_msdos
 insmod ntfs
 set root='(hd1,msdos1)'
 search --no-floppy --fs-uuid --set=root 28E826DCE826A7D0
 chainloader +1
}

(see the thread for more information)