Many possibilities of what it could be, go into the game, minimize and check the cpu usage or how much memory is being used, if less than 100% prolly trojan downloader or other program that messes with DirectX.
A DXDIAG wouldn't hurt either. also make sure no other programs besides those normal in the background are working, Internet, Word, Gimp, etc all slow down your computer, the problem may be a driver as well.
