1. Turn it off and on again.
2. Turn your volume control up.
3. Plug your speakers in.
4. Make sure your PC sound isn't muted.
After that, I'd reckon it was down to codecs, which are little pieces of black magic that you need to load into Windows Media Player and its ilk to make videos play properly. If you don't have the right codec loaded up, your videos either won't play, or they won't play properly. Try loading up DivX or RealPlayer or even the dreaded QuickTime and see if your fillums play through one of those.
Yes you might have deleted something during your "blitaz" [sic] recently. I doubt it would be an audio driver, else you wouldn't be getting any sound at all off your pooter. Er... are you getting any sound at all off your pooter?
BTW, I don't normally say "pooter". I don't know why I'm saying it now. I'm not drunk.