PC graphics problem

System specs:
Windows Vista™ Home Premium
Version 6.0.6002 Service Pack 2 Build 6002
System Type x64-based PC
Processor AMD Phenom(tm) II X4 945 Processor, 3000 Mhz, 4 Core(s), 4
1GB ATI HD4670 Graphics accelerator
Installed Physical Memory (RAM) 4.00 GB
Total Physical Memory 4.00 GB
Available Physical Memory 2.78 GB

when I play games such as The Sims 3 or Harry Potter and the deathly hallows, the graphics are really strange. How can I fix this problem? Thank you!

image: img811.imageshack.us/img811/2972/38004169.jpg
 
How long are you playing before this starts? it looks as if it may be corrupt or overheating memory.
 
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