I ghost machines several times a month... but that's my job. I use OSX at home because it's hassle free and I've never had to reinstall it in the 3 years I've used it thus far.
But really, if you know what you're doing, then XP is good for years on end. We have many computers here which have been used DAILY for
5 years straight without much hassle. Granted, they are running enterprise grade AV software. However there was no disk maintenance, so these drives are 35% to 70% fragmented (yes, I have never seen fragmentation this bad) so I just implemented scheduled defrags.