Please post a dxdiag report. You can do so by:
Click Start->Run-> type in dxdiag and click OK.
Once it finishes, click the Save All button, give it a name, then open that file up in Notepad, copy+paste everything to pastebin.com or similar, and post the link here (please do not paste the report directly here as very long texts like that can make the page difficult to read).
This will tell us about your hardware and driver versions which may help pin the problem down.
