Computer reboots for no reason

[Jersey General]

Installed Civ4 today, have had no problems actually running the game. Then, at random times in the middle of games, my computer simply shuts down (rather than reboots--ignore the thread header). Period. No warning, no error messages, no notice from Civ4, nothing. Anyone else had this problem and found a fix? I'm using the 1.52 patch.

Specs:

Toshiba Tecra S2 laptop
Pentium M 740 1.73 Ghz processor
Windows XP Professional w/Service Pack 2
512 MB RAM
NVIDIA GeForce GO 6600 graphics card
64 MB video RAM
24x CD-ROM drive