Media playback problems in WMP 11 (Vista Ultimate)

This is both a recent and not-so-recent problem on my computer. I installed
Vista Ultimate back in May and everything has been fine until about a month
ago, when I started to have audio "shorts" where the sound would suddenly
clip out. I'd go into the playback devices menu, switch the playback format,
and test the audio; that was enough to fix the problem. Then, two days ago,
I heard another audio "snap" that usually accompanied the audio short. I did
my usual fix and it seemed to work; the Windows sounds came back, but now WMP
11 refused to play any encoded music file (mp3 or wma) or video file (.mpg or
wmp), although it would still play nonencoded files from CDs. Furthermore
Winamp still plays all the files, although video is now somewhat choppy. I
also recently upgraded my NVidia drivers to 163.69, although I don't see how
that could affect audio playback. If anyone has an idea as to what happened
here, I'd be glad to hear it. Thanks.