Game Loading Rebooting 6600GT

Hi all, i recently put together a nice new system. (ASRock K8 Combo-Z Mobo,
6600GT vid card, 1GB (2x256 Ram)) Latest version of directX, latest drivers
for everything (except maybe AGP drives, got them off mobo disk), first
thing i done to try out my new beast was install far cry, ...and bar a few
graphic glitches (that were mostly fixed by patching game, it worked fine.
While downloaded latest firewall software the computer got virus, so i
totally f-disked and started again. This time though, when i run far cry the
game causes the system to re-boot about 1/10th into loading level! So very
confused, i tried Doom 3, ....which loaded up the level fully, but as soon
as it started to go into the in-game, u could see the first frame, ..and it
would also hang, and cause the system to re-boot! I tried loaded a few doom
3 levels, and the only one that worked, loaded up and played was the first
hell level, i'm guessing that worked because theres less stuff drawn. I
tried lowing the AGP speed from 8x to 4x with no joy, and disabling and
enabling fast writes....still no joy.

So, do any of you clever people out there have any idea what might be
causing my system to get re-booted when loading up games?!?!?!, other than
this the system is nice and stable. I don't believe this to be a hardware
fault as pre-fdisk the games loaded up and played fine, ....but maybe you
guys know better.

Any advice very much appreciated,
Thanks,
Dan