Games Instantly Shut off computer

When I load either Fear combat or guild wars, my computer instantly shuts off, which is kinda annoying.

The game will launch, I see a black screen, it will begin to load, then I hear the click and sound of my hardrive shuting off, and my monitor goes into idle mode.

Then my computer automatically re-boots and runs normally, and tells me that it recovered from a serious error.

I'm currently running

a HP Pavilion Meda Center TV m7480n Desktop PC, 2 gigs of ram, dual 3.0 prossessor, Nvidia GeForce 7300 LE video card, and XP pro.

FEAR/GW was working fine until I loaded my restore point from november, due to some problems with using Premiere Pro.

I'm having some trouble finding the solution, as I don't know the problem.

Any help or suggestions would be much appreciated, thanks.

if this helps heres a list of games that work for me:

Oblivion - yes

Quake 3 - yes

Enemy Territory/Wolfenstein - yes

Halo - yes

Dungeons and Dragons Online - yes

HL2 - yes

Guild Wars - NOPE - game crashes after update

and FEAR combat: crashes once I select a gun

any ideas? thanks