Moved to support forum.
Without more details of your machine, it's difficult to provide advice. Probably the first thing to try is the dual core fix - see the thread I have linked below:
http://forums.frontier.co.uk/showthread.php?t=374
If that doesn't solve the issue then you need to post your dxdiag report. To do this, click Start, then Run, type dxdiag and click Ok, then click Save all information and copy and paste the saved text file into your reply to this thread. You will probably need to split it over a few posts as the reportsa tend to be quite long.
Michael