1. Keep data on a separate hard drive - then it's easy to format C and start again.
2. Use something like "Ghost" or "Driveclone" or "Driveimage" to back up your entire C drive once everything on it is clean and shiny - then install all the usual crap like Office and Avast and Spybot Search and Destroy and Ad-aware and Brava and your media player and Acrobat Reader - then make another clone.

Then you can format C and re-install in about 15 minutes flat.