And, you used that computer on the Internet regularly? What type of Internet connection do you have? Dialup, DSL, cable?
Again, If you have important files that have not been backed up, you should back them up ASAP, if you still can.
Exactly what McAfee product do you have installed? Is it just anti-virus or does it include other Internet security features?
Have you ever re-formatted the hard drive and re-installed your OS? Apparently, you either did that when you upgraded a hard drive, or you had someone else do it. Do you have your Windows CD?