I generally reinstall my PC's OS every 6 months on average. This is now because I use Ubuntu Linux which has a new release every 6 months, but I generally reinstall XP about that time too, because usually by then I've got some laggy startup programs that I really don't need, one or two instances of spyware that I don't bother to erase, some messed up system settings, etc. My PC uses recovery CD's so I don't worry about limited activation retries.