I guess most of the time. (I'm really not that nerdy.)
I work with Microsoft licensing and I'm working on a $40k agreement with them right now. I'm covering all the bases with them and finding out what I can and can't do.
XP (Home or Pro) Retail, Upgrade you can use it in any machine you want but not in two at the same time.
XP (Home or Pro) OEM is tied to the original machine (although you can change components as long as one original component remains --example: you can change the video card)
Only Office 2003 can be activated in 2 machines one desktop and one laptop at the same time.