computer restarting on sims 2

When I start the sims 2 game the game gets to the screen where you start to see the town. For example in Pleasantville, you see the bridge or maybe the whole town and all of a sudden the computer freezes and then restarts. Sometimes it does work and then I can play. And sometimes when I am playing it will restart.
My son built my computer and this is what he put in it.

processor: amd athlon 64 processor 3000+, mmx,3dnow,~2.0ghz

memory 1024 mb ram

video card: ati radeon 9600/X1050 series

sound card: realtek AC97 audio

directx version: directx 9.0c is installed.

I did the memory test at crucial and this is what i got:

Maximum Memory Capacity: 768 MB
Currently Installed Memory: 1024 MB
Available Memory Slots: 1
Number of Banks: 3
Dual Channel Support: N.A.
CPU Manufacturer: AuthenticAMD
CPU Family: AMD Athlon(tm) 64 Processor 3000+ Model 12, Stepping 2
CPU Speed: 2009 MHz

plus my son just installed a new 200 gb hard drive.

1 out of 20 times when I start the game it will play ok. Most of the time it restarts. In the town of Veronaville I can't scroll around the town, it just stays stuck on the one screen, but I can pick a house or lot.

All other programs work fine on the computer. Sims2 is the most graphically challenged game I have ever played. I would love to just turn on the computer and play without restarting for 2 hours and hoping to get a chance to play.

Would appreciate any help,
Thanks,
Cybeeb