1) Perform clean boot (Check the link for performing clean boot).
2) Enable half of the services.
3) Determine whether the problem returns.
4) Enable half of the startup items.
5) Determine whether the problem returns.
6) Repeat the above steps until you find out which service or program is causing the issue
Note: Please ensure that the computer is set to start as usual by following the step 7 from the article: How to troubleshoot a problem by performing a clean boot in Windows Vista or in Windows7:
Please provide us more information so that we can assist you in a much better way.
Hope the information helps. Please post back and let us know.
Microsoft Answers Support Engineer
Visit our Microsoft Answers Feedback Forum and let us know what you think.