audio not sync with video.

hey everyone, i seem to be having a little problem with audio playback with my computer. if i download a movie clip from the web, when i try to play back on pc, the sound is streemed out faster than the video or slower... anyone know what could be the problem? i have core 2 duo at 3.2, 2 gigs of ocx platimum ram, lots of storage, creative x-fi platimum, and ati x1900xtx video card. tried the same burnt dvd in another pc and everything works fine. a dvd movie from video store works fine. seems to be problem with avi files...