Need Help, Now!!

[Emperor]

I installed WinFast PX8800 GTX TDH(NVIDIA). Every PC Games worked like a charm and impressive preformence. This is where the tides got upside down. I was playing Medieval 2 Total War Campaign and I was soo excited about the Graphics. The Campiagn works but after, When I was attacking a castle, while loading is fine. But when the battle starts and I start moving the mouse a bit and it was realistic. But then about 5 or 6 seconds the computer closed or shut down by itself. But the CPU still flashes but the Moniter(Screen) stopped Responding.

Who is to blame? Nvidia or should I change the Screen Resolution, and I installed 2GB Memoryto heat up the preformence.

But I think it's the Screen Resolution. I hired two computer engineers to install the Card. Please, answer my problem. I want to play before Kingdoms is Released.

[Emperor]

This might help you?

My Computer:-
Model: Aspire T660 but with Intel Pentium 4.
Windows Vista Ultimate.