Rebuilt from scratch here.  In my case, as an IT guy, I actually don't mind doing it and it gives the system a nice fresh start.  Since I have all of my audio files, my documents, and my VST instruments on a separate drive, I will likely make an image of the system as it is now for a quick rebuild to this nice clean state.
If you have had a system built for a while, I'd recommend a completely scratch build, especially if you are like me and have a system used for a few different purposes - in my case, web design/development along with the music stuff.  Makes for a nice new environment.
 
Also, going from Windows 7 32bit to windows 8.1 64 bit is a great change as well - especially with 4gb that I wasn't able to fully utilize until now.