I'm on dapper amd64, so I don't have win32codecs. I've tested with DivX3, DivX5, and Xvid files, and all play back fine.

Could you paste the terminal output from gmplayer (open a terminal and type "gmplayer path/to/my/movie.avi") and the output from "file path/to/my/movie.avi"

It will look somwhat like this

jamie@doomguy:~/media/movies$ file "Hitler, Rise of Evil (TVrip Part 1) -- [imdb tt0346293 | Biography,Drama | English].avi"
Hitler, Rise of Evil (TVrip Part 1) -- [imdb tt0346293 | Biography,Drama | English].avi: RIFF (little-endian) data, AVI, 352 x 288, ~25 fps, video: DivX 5, audio: MPEG-1 Layer 3 (stereo, 44100 Hz)


and gmplayer output is


jamie@doomguy:~/media/movies$ gmplayer "Hitler, Rise of Evil (TVrip Part 1) -- [imdb tt0346293 | Biography,Drama | English].avi"
MPlayer 2:0.99+1.0pre7try2+cvs20060117-0ubuntu8 (C) 2000-2006 MPlayer Team
CPU: Advanced Micro Devices  (Family: 15, Stepping: 0)
CPUflags:  MMX: 1 MMX2: 1 3DNow: 1 3DNow2: 1 SSE: 1 SSE2: 1
Compiled with runtime CPU detection.



xscreensaver_disable: Could not find XScreenSaver window.
91 audio & 204 video codecs
Linux RTC init error in ioctl (rtc_irqp_set 1024): Permission denied
Try adding "echo 1024 > /proc/sys/dev/rtc/max-user-freq" to your system startup scripts.
Opening joystick device /dev/input/js0
Can't open joystick device /dev/input/js0: No such file or directory
Can't init input joystick
Setting up LIRC support...
mplayer: could not connect to socket
mplayer: No such file or directory
Failed to open LIRC support.
You will not be able to use your remote control.
[GUI] Adding video filter: pp
Playing /home/jamie/media/movies/Hitler, Rise of Evil (TVrip Part 1) -- [imdb tt0346293 | Biography,Drama | English].avi.
AVI file format detected.
VIDEO:  [DX50]  352x288  24bpp  24.995 fps  832.3 kbps (101.6 kbyte/s)
==========================================================================
Trying to force audio codec driver family libmad...
Opening audio decoder: [libmad] libmad mpeg audio decoder
AUDIO: 44100 Hz, 2 ch, s16le, 224.0 kbit/15.87% (ratio: 28000->176400)
Selected audio codec: [mad] afm: libmad (libMAD MPEG layer 1-2-3)
==========================================================================
xscreensaver_disable: Could not find XScreenSaver window.
Opening video filter: [pp]
==========================================================================
Opening video decoder: [ffmpeg] FFmpeg's libavcodec codec family
Selected video codec: [ffodivx] vfm: ffmpeg (FFmpeg MPEG-4)
==========================================================================
[PP] Using external postprocessing filter, max q = 6.
Building audio filter chain for 44100Hz/2ch/s16le -> 0Hz/0ch/??...
alsa-init: 1 soundcard found, using: default
alsa: 44100 Hz/2 channels/4 bpf/65536 bytes buffer/Signed 16 bit Little Endian
AO: [alsa] 44100Hz 2ch s16le (2 bytes per sample)
Building audio filter chain for 44100Hz/2ch/s16le -> 44100Hz/2ch/s16le...
Starting playback...
VDec: vo config request - 352 x 288 (preferred colorspace: Planar YV12)
[PP] Using external postprocessing filter, max q = 6.
VDec: using Planar YV12 as output csp (no 0)
Movie-Aspect is 1.22:1 - prescaling to correct movie aspect.
VO: [xv] 352x288 => 352x288 Planar YV12
[mpeg4 @ 0xbadbb0]frame skip 8ct:  0.000   1/  1 ??% ??% ??,?% 0 6
alsa-uninit: pcm closed 0.000 ct: -0.003  53/ 53  7% 10%  1.0% 1 6

Exiting... (Quit)