this has happened on one or two of my games (legit games) - it's really got nothing to do with pirated or not as far as I know. In the first case, Battlefield Vietnam, it was because I had the same card as you and had the graphics on medium - the fact is it's a pretty crap card and is unable to perform well. I turned the graphics down and it was fine.
The other time was the demo for Rome: Total War. (This was after I upgraded to a radeon 9800). However, upon getting the real game, they had applied a patch or something and it fixed it perfectly. Something to bear in mind as well is your computer temperature, as tonnes of errors can be attributed to overheating processers etc.
If possible, get your case / cpu temp when idling (just in windows, or in the bios), then again straight after you've been playing games for a good half hour at least.
The only solution I can recommend to you is to first try and turn the graphics down as much to reduce the load on the graphics card. Second, if that doesn't work, is a costly upgrade to a better card. Unfortunately this will likely set you back about £70 - £80.
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