And while it is a bit extreme to think this - a lot of times when you bring a PC into a repair shop (especially doing it yourself) it gets more damaged in the process (or damaged in the first place). Anything from oily fingers to a random static shock could ruin a stick of ram (oily fingers causes RAM to overheat).

I sorta have to agree with Immortal and say it's memory related... but I'm no expert.

If you havent upgraded to SP2 yet, I would suggest you do so as it manages a lot of memory quirks fairly well. While not a "fix," it's a work around to "bad" hardware. It will at least help you get through until you can replace the sticks (if they are indeed the problem).

Again, best of luck,

Amejin