I generally only start from scratch when changing major versions. I used XP for something like 8 to 10 years. I've heard that you're "supposed" to reinstall Windows every 6 to 12 months to keep it working fast, but my install didn't start playing up until a few months before I upgraded to 8.1 (and that was only because XP support was being discontinued). It must have known the writing was on the wall.
