Audio codec problem?

I've been experiencing some audio problems recently on one my systems.
The problem only occurs on Windows video files (AVI, WMV, MPG, etc.),
but not on Quicktime video files (MOV). I've tried several different
media players, from the default Windows Media Player, to Media Player
Classic, RealPlayer, Nero Showtime, and various other media players
lying around that were packaged with various things.

The symptoms are always the same. The video plays, but the audio doesn't
after the first time. So for example, you can play any video file with
its video & audio the first time after reboot, but when you try playing
it a second time (or playing any other video file for that matter), the
audio doesn't play anymore. Windows Media Player starts and exits
saying, "You're running low on memory, quit other programs and try
again", followed by the "Close" button. Yeah, right I have 1GB of RAM,
and 3.5GB of swap! Media Player Classic just aborts completely.
RealPlayer plays the video, but there's no sound. Other media players do
the same sort of thing too.

Now, I think it's an audio codec problem because Quicktime player plays
its own files without problems, since it has its own native codecs
built-in. Also audio files aren't affected if you play them through
Winamp, which also has its own internal MP3 codec. But try to play the
same MP3's through Windows Media Player, and no sound comes out, because
it's using the default system codec.

I'm not sure if this is related but the problems seem to have started
after a recent Microsoft major patch update. I was thinking maybe the
update to WMP 11 was causing it, so I uninstalled it and reverted back
to WMP 10, but that didn't cure it. I've also updated the soundcard
drivers (Hercules GameSurround Fortissimo III 7.1) to the latest
available, but that didn't do anything. Windows XP SP2, of course.

How would I go about debugging the codec? Changing default codecs, etc.?

Yousuf Khan