Thanks for your help guys!
Heres what we have done:
BIOS update for our Biostar T-Force 945-P
Installed new 650 Watt power
Used a driver cleaner to get rid of old Nvidia drivers
Monitor still freezes, followed by the computer freezing, requiring a reboot.
This only happens on FPS games such as Counter-Strike and Bf2. But it will not freeze on the RTS games such as Company of Heroes.
Any more ideas would be appreciated.