Hi,
How does this issue begin to occur? Can you associate with any software\hardware change?
I would suggest having a look whether you can reproduce the problem in safe mode.
Start your computer in safe mode
http://windows.microsoft.com/en-US/windows-vista/Start-your-computer-in-safe-mode
If behavior doesn’t persist in safe mode, it indicates that a third party program or service is preventing you from performing the action as supposed
to. During Safe Mode, please launch your antivirus software and perfrom a system scan to make sure the system is clean. Then continue to troubleshoot the issue in clean boot.
Clean Boot prevents software conflicts by preventing the startup items and non Microsoft services from loading. This way you can safely follow the process
of elimination to check which application is causing this behavior.
Refer:
How to troubleshoot a problem by performing a clean boot in Windows Vista or in Windows 7
http://support.microsoft.com/kb/929135
Note: Ensure that you put your computer back to normal startup mode after completing the troubleshooting steps. Steps are available in the article given
above.
Regards,
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