Having problems with Product Key

Hi I purchased my PC four years ago and it came with the Windows Vista installed. I have been using it for the past 4 years without any problem but now a few days back I get this message
"There is a problem with your license Description: The Security Processor reported a system file mismatch error. Validate your product online."
I have a valid OS disk that comes with my PC and a valid product key and I did check that it was activated when I bought my PC. I have tried everything to resolve the problem even reinstalled my OS, still I get the same error after sometime. Need help.
