There are two user directories (with your parts and the parts database) which don't get touched during an install (to not affect your sketchs during upgrades). On Windows they are in
c:\users\username\AppData\Fritzing\roaming\Fritzing (which is a hidden directory so you need to enable hidden directories in explorer) and
c:\Users\username\My Documents\Fritzing (where username is your windows id)
If you don't have any parts or sketches you want to keep you can just delete those two directories and Fritzing will receate them, or you can move them aside by renaming them if you wan to keep something in them.