I agree it seems to have a mind of its own. I have a copy of xp pro that i have used several times on different macchines , just have to ring up for a new activation number.
They have strated grilling you a lot more these days about how many macchines you have it running on etc. Just one guv.
I had also heard that with service pack 2 (oem versions) that would not let you activate if compleete new system , but in last few days I have installed medica centre edition with service pack 2 bulit in oem version on two totally different machines both connected to the net and both activated over the net, so go figure