Backup iTunes (store purchases, iphone backups, playlists) for Windows reinstall

The easiest way is to go to C:\Users\<Your Username>\Music\ and copy over your ENTIRE iTunes folder (sometimes called iTunes Music) to a USB drive or some other backup location. When you reinstall Windows, install iTunes, but don't open it. Go back to that directory, delete the iTunes folder the installer created and move your copied folder back. Everything will be identical to how it was before you reinstalled (save for, of course, iTunes settings).