Happens to me 1-2 times in a month and it's very annoying.
As i'm playing my interface gets bugged i close the game and when i doubleclick the exe again it starts downloading instalation and overwrites the existing dat file. I cannot fix it and the only way seems to be reinstall which is a lot of downloading even with the instalation cds.
I agree with EternalTempest. This sounds like a RAM problem to me, but could also be a hard drive problem. I've also seen bad IDE drivers do things like this. I'd start out with Memtest86+ as ET suggested and go from there.