Help please!!

Hi,
So I've started playing The Sim 3,
but it starts to lag, then freeze and then shut off my computer (and I do not understand why!)

It starts off with the music jumping and crackling, then it lags, then it freezes and then it shuts off my computer (well the computer restarts itself but the screen says "no signal")

I have windows 7,
here's my computer's system:
Manufacturer: Packard Bell
Model: ixtreme M5722
Processor: Intel(R) Core(TM)2 Quad CPU Q8300 @2.50GHz
Memory (Ram): 6.00 GB
System type: 64-bit operating system.

My graphics card is a Geforce GT220

I really need help, as I love sims. But it really annoys me when it shuts off my computer.
I'm not really much of a computer person, so could you please give me your answer in the simplest of forms? D: