It sounds like you have problems with your forceware, if you had any previous drivers on your drive you should remove them or they will conflict with one another. Once you're sure of that reinstall your nVidia drivers.
Also, it may be a weak PSU that causes freezes, but the former is the obvious main culprit.
Intel C2D E6320 / AMD Athlon X2 3800+
Gigabyte 965P DS3 / DFI nF4 Ultra-D
2GB OCZ Gold PC2-6400 / 2GB OCZ Gold PC4000
eVGA 8800GTS 320MB / eVGA 6800GS 256MB
150GB Raptor / 74GB Raptor
2x500GB / 320GB
OCZ GameXStreme 850w / OCZ StealthXStream 600w