I've been having the exact same problem - mine seems to be restricted to nighttime. I can use the computer all day, then in the evening all of a sudden I'm freezing and spending more time doing reboots than I'm getting work done. Though I get daytime crashes, but it seems like a 10-1 ratio.
In my case, it appears to be restricted to using After Effects (cs3). My suspicion is that it is drive related, but I went through every drive I have in the Cache settings. I can run Photoshop or Itunes all day and night and never see a single freeze.
I also theorized its possibly a font issue, as when I was working in a type-animation comp (type only, no other elements) all of a sudden the crashes went through the roof - and Macs/OSX has the same problem - one corrupt font can cause all kinds of bad behavior in the system (and Macs seem particularly good at corrupting their fonts).
Of course, replacing the fonts in question didn't fix a damn thing.
So my last ideas - c: drive going bad (though its a brand new XP install, its a fairly old drive) or an Open-GL issue. The last thing I'm going to try is to replace the OpenGL drivers, then I'm going to drop the computer in the ocean. That will fix it once and for all.
Anyway, you might want to look into your OpenGL drivers, or whatever video drivers you happen to be using...