game opening and lag error
 

I just bought my game off steam and my game is lagging alot I could play it for a week with lag then when I tryed to play today an error message popped up saying that the the screen resolution was to low but it is not I have installed and uninstall plz help

amd Athlon dual-core ql-64 2.10 ghz

3.00 gb of ram

windows vista 32 bit operating system

nvidia Gforce 8600m g