Do you have the latest Windows updates? There was a bug a few months ago that had an effect on that type of thing. MS issued a patch a while back. Other than having the wrong driver installed, I can't help you. Click start, run, and type dxdiag in the box and see what it tells you.
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If your equation requires either a complex combination of variables to get the desired result OR you can't seem to get the result regardless of the variables....it ain't your variables, it's your constant.