Hi,
Check if there is information in event viewer that might help us in troubleshooting the issue:
http://windows.microsoft.com/en-US/windows7/What-information-appears-in-event-logs-Event-Viewer
Step 1:
You may try to boot in safe mode and check if the issue persists. Only the basic files and drivers necessary to run Windows are started. If an existing problem doesn't reappear when you start in safe mode, you can eliminate the default settings and basic device drivers as possible causes. To boot in safe mode follow the link.
http://windows.microsoft.com/en-US/windows7/Start-your-computer-in-safe-mode
Step 2:
If the issue does not occur in safe mode then boot to the desktop in normal mode and check which of the third party program is causing the issue. To configure your computer to clean boot follow the link given below.
How to troubleshoot a problem by performing a clean boot in Windows Vista or in Windows 7
http://support.microsoft.com/kb/929135
Note: After you have finished troubleshooting, follow step 7 in the article to boot to normal startup.
Hope the information was helpful