Post info on your computer and settings you are using.
Run Dxdiag and post the first/top third of the report. Tells as what stuff you have in your computer and a bit about how it runs.
When you check your processes, how many are running when you play? On a fresh boot, watch the percentage in the corner. It should jump to 1% and drop back down with the computer at idle.
What game settings are you using? Texture size 512,more, less? How much "eye candy do you have turned on in the setting ground clutter, shadows and such?
The more info you can post here the easier it will be to help you find some answers.