Another Microsoft tax. Why so many accept and put up with that is beyond me even after paying for antivirus software year after year after year. You should dump that swiss cheese for security garbage the first chance you get.
Your iTunes library should be included with your computer's backup along with all other important data stored on an external drive or some other external media used for backup purposes only. This is important regardless the platform but a number 1 priority with Windows due to a Microsoft tax - constant virus, adware, and spyware concerns.
http://support.apple.com/kb/HT1751
This link includes instructions for consolidating your iTunes library. Give that a try to see if it finds the music files on your computer's hard drive that are missing from your iTunes library.