Playing a game exits or restarts computer

[orphan]

So I'm trying to play Civ 4 and whenever I try to load a saved game, it gets done loading, then completely exits out of the game and leaves the screen at a smaller resolution. This also happens sometimes when I try to start a custom game and sometimes it restarts my computer. It also exits out of the game lately as I'm an hour into the game. When it's waiting for the AI to move, it will exit out of it. This has only recently started happening; when I first installed the game a few weeks ago, it worked fine and I could load saved games.



Specs:

WinXP
Intel P4 2.0 ghz CPU
Gigabyte Technology Co., Ltd. 8IG1000MK mobo
512MB RAM
LITE-ON 52x CD-R
120GB Seagate Barracuda HD
60GB Western Digital HD
Radeon 9200 Vid card