I'd seen something like MS having a four month window on being able to reinstall without any crap. Crap being having to call the automated tech.
I'm juggling a couple copies and although I don't want to sell the Home version, I want to install Pro and change the Home version to a lesser machine. Anyway, right now, I want to swap hard drives around to copy and archive data. (2) WD1200JB 120GBw/8MB cache, (2) Maxtor 80 GB and (2) IBM 60 GB hard drives. I've got one single (the same) version of XP Home loaded in each pair and have started to experiment. Two pairs switched out within 30 days after activation and no problem.
If you have an OEM version that came with a particular brand computer it probably won't work on a different brand computer anyway, if that's your situation, beware.
The retailers stipulate up front that, there are to be no returns on any XP OS because of product activation.
I don't know for sure, but I think after a certain time period it can be reactivated on a different computer.