Backup everything, make sure you have all the drivers for your system. Export and save your email address, bookmarks, etc. Stick it all in a folder, burn it to a dvd (or 2)
Boot with your OS Cd in the tray, repartition, reformat, reinstall. Once windows is done loading don't forget the motherboard drivers first, then video, etc.
Then reload software.
If your any good, 4 - 6 hours later you'll have a spotlessly clean, lean AH flying machine.
Just take a day and do it once a year.