I apologize for the length of this post, but I wanted to be thorough and tell it my way.
I've got this problem with my laptop (it's a Sony Vaio PCG-K23 - PDF of the specs from Sony's customer service
) that confused me a bit recently. All of a sudden, one day, I lost the ability to display video on my computer. I could still do YouTube and Google Video, cuz they're basically just flash, but any video of any file type I tried to play on my computer just showed up as blank. It played and I could hear everything, I just couldn't SEE anything. There was also sort of a static effect on my screen. Every once in awhile, there'd be this line that would flash on the screen. It started happening pretty often, unless I had a game running full-screen. I tried uninstalling and reinstalling my codec packs, but that didn't work at all. After some other crap happened the next day, I just reformatted and everything worked fine again. Until just this afternoon, when I was installing some Windows Updates and I decided to run a video again and once again, it was black, and now I've got that flashing line thing on my screen again. So I'm pretty sure it was something that Windows Update installed. I just removed two (the only ones I could find) updates related to Windows Media Player and installed Windows Media Player 10 ('cuz I hadn't done that yet). The only other thing I can think to do, is to reinstall the display drivers after Windows Update has had its fun with my computer. But if anyone knows anything more about this subject, seeing as how I'm really not sure my idea's gonna work, I welcome other people's help.