1. Do you receive any error message?
2. Have you made any changes to the computer prior to this issue?
There is a possibility that some third party programs installed on the computer is causing the issue.
You may try to boot the computer in safe mode. Refer the following link.
Start your computer in safe mode
If this issue doesn’t occur in safe mode then you may perform a clean boot. Refer the following article.
How to troubleshoot a problem by performing a clean boot in Windows Vista or in Windows 7
NOTE: Once you are done with clean boot, please follow step 7 in the article to set your computer back to normal startup.
You may try to perform SFC (System File Checker) scan on your computer and check. Refer the following article.
How to use the System File Checker tool to troubleshoot missing or corrupted system files on Windows Vista or on Windows 7
I would also suggest you to scan your computer with Microsoft Safety Scanner, which would help us to get rid of viruses, spyware, and other malicious software.
The Microsoft Safety Scanner is a free downloadable security tool that provides on-demand scanning and helps remove viruses, spyware, and other malicious software. It works with your existing antivirus software.