Could be a hardware problem, or maybe just a driver problem. Have the side of your case off while playing, see if that helps. If not, make sure you have the latest catlyst drivers for your Ati Radeon card (
found here:
Catalyst 5.4) .
If none of that helps, see if it crashes on other high detail games like Doom 3, Far Cry, Need For Speed Underground etc etc (something that will have ur comp sweating). If it does, and the side of ur case if off, and your drivers are all updated, it could possibly be a power shortage.
Have you changed any hardware lately?
I play CS: Source and HL2 DM quite often, and its never crashed and rebooted one me. Also i havent had that friends list working for about a month now :/.