Step 1: Boot into safe mode with networking and check if the issue persists.
See Advanced startup options (including safe mode) for instructions on booting into safe mode with networking.
Step 2: If the issue does not occur in safe mode,boot your system in a clean boot state (in normal mode) to troubleshoot the issue.
See How to troubleshoot a problem by performing a clean boot in Windows Vista or in Windows 7
Note: After troubleshooting, make sure to put the computer to start as usual as mentioned in Step 7 in the above KB article.
Method 2:
Perform system restore to a restore point prior to the issue and see if that fixes the issue.
Also see: System Restore: frequently asked questions