Have you made any changes to your computer?
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.
Perform System File Check to fix the corrupted system files and try to run Norton.
Refer the following link to perform System File Check.
How to use the System File Checker tool to troubleshoot missing or corrupted system files on Windows Vista or on Windows 7
Note: It may ask for windows DVD to fix and to enable SFC to make more than minor repairs. Some files saved on your computer might get deleted when fixing corrupted files.
I would also suggest you to follow the link and check.
I would also suggest you to contact the Norton Tech Support.
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