I built new systems for my girl & I for the holidays; had a lot of trouble with mine & still haven't resolved the oddest one:
Sometimes when I exit from a game, the system restarts without warning. To me this suggest a video card problem, or a power supply one.
Both check out OK, though.
(& by game, I mean something that more or less takes control of the system, like Battlefront, or Half Life 2, or whatever, not Minesweeper.
I should mention that the root of most of my problems was the 160 GB drive that did not cooperate on IDE 0 at all. It's on IDE 1 now.