Steam Crashes

[Bac-Talan]

Right, so here's the problem:
I, several times now, tried to use Valve's Steam program to run games, namely Portal: The First Slice and the Half-Life 2 demo. They install perfectly, run, and load up to the main menu screen, at which point my computer reboots.

I've tried all I can think of, the usual reinstalling, redownloading, etc. And I still have this problem.
I don't believe it is a lack of hardware, as I can run Crysis (not terribly well, but it runs ) and haven't encountered a problem like this before.

I'm considering seeing if I can get the Orange Box, because I've heard a lot of good things about it, but not if I can't get this resolved.
Anyone heard of this problem? If not, any suggestions anyway? Similar issues?
Thanks.

[Precise_Moon]

have you purchsed the games through STEAM........

Better to buy from a retailer then you have the disk and i just use steam like xfire to see who is were...Have you tried shutting some programs down which are not in use whilst on game maybe you are getting some conflict there