What to do when Windows XP gets really slow?

Well, this is my main complaint about this licensing crap. What can
you do when you reach the point of needing to reinstall the OS because
it is just creeping along at a snail's pace? It used to be a pain in
the neck, but every year or so (depending on your usage) you really
needed to clean off your HD and reinstall, but is it possible to
reactivate it?

I would really like a clean start again.