computer problems while gaming

Hey guys.. I have a problem with my computer. When I play my game (Sims 3) on my computer after an hour or two it shuts off by itself. I am not sure what's wrong with it.. Can someone please help...

Here are my computer specs:

Operating System
MS Windows 7 Ultimate 64-bit
CPU
AMD Phenom II X2 555 24 °C
Callisto 45nm Technology
RAM
4.0GB Dual-Channel DDR3 @ 669MHz (9-9-9-24)
Motherboard
ASUSTeK Computer INC. M4A785TD-M EVO (AM3)
Graphics
W2053 @ 1600x900
ATI Radeon HD 4200
Hard Drives
733GB Western Digital WDC WD7500AADS-00M2B0 ATA Device (IDE) 29 °C
Optical Drives
ASUS DRW-24B1ST ATA Device
Audio
VIA High Definition Audio

 

What are the recommended graphics cards at the back of the game cover?
Does it only shut down the game, or your whole computer?
has it been doing this since you have been playing or only recently?
Is that graphics your onboard card?
Are you playing online or offline?

I have not used an Windows 7 operating system as of yet so i dont think I can be much help.

so far it sounds like your graphics card, try look for any driver updates
how many watts is your power box?

Try setting all graphic options to low or detect automatically in the game

If the whole computer shuts down, you have a problem, windows forcibly shuts down to protect files from getting damaged