Computer Problem: I have NO clue what's wrong...

So, I've built a couple systems before, 2 with XP, one with Linux, and thought I'd try out Vista...build goes fine, only minor issues with the hated 775 retention pins. But anyway. I install Vista, having the problem of trying to install 64-bit Vista onto a system with more than 2GiB of RAM installed, removed two modules, worked fine, installed the 3+GiB RAM patch, then replaced the two modules. Computer worked fine for about two days.

Then I started noticing issues where programs I started would essentially be blank. The frame of the window would open, but there would just be a white square where any text should be, or it would just not open at all. Note that I've done no overclocking whatsoever. I thought I might have something screwed up with my RAM, so I tested each one by one, no dice. I can make the computer crash by running 4 instances of Prime95 (Core2Quad system), and the crash occurs in about 20 seconds.

Any help would be greatly appreciated, I'm at my wit's end.

Does removing the RAM change things?
List your parts so we can help further.