more on mbr has been changed... a virus infection may not be the case here at all, but the more i search, the more i encounter this as something to consider. hopefully you have a good av program installed that you can scan with, or go to symantec's site and i believe you can scan for free or even download their av trialware.
"Q: What does "Master Boot record has been changed! Do you want to restore original (saved) Master Boot Record? (Y or N)" mean?
Answer: This message is displayed if changes in the master boot record are detected. Changes to boot sectors are one of the most common signs that a computer is infected with a boot virus. Some file viruses, storing their counters in boot sectors, can also trigger this message. Note that if you change the operating system installed on the computer the boot sector may also change and trigger this message. If you know this to be the case this message is nothing to be alarmed about. If you know of no reason why the boot sector should have changed you should investigate the reason for it."