Windows Software Protection Error Notification

Hey guys, I have been using Vista for almost a year now and did not come across any problem. Just few days back, my system failed to load the Vista welcome screen. After that, I got the following error:

“An unauthorized change was made to Windows.
You will no longer receive notifications, including those about your license or activation. Use the link below to find out how to fix your system.
Error: 0xC004D401
Description:
The security processor reported a system file mismatch error.”
I have tried deleting all software except the ones which were on the PC when I bought it, but this measure does not seem to have brought about any changes. Please advice as to what I should do about this.
Waiting for reply.
