I've actually spoken to a live rep from Microsoft about this before, I transferred my XP to another computer and called Microsoft, I told them I upgraded the parts in my computer and once I was given a new authorization code (or whatever it is they give you when you call) I asked how many computers I could use it on in my household, the person told me you can run it on one desktop and one laptop, I've never tried it though because I've never had a laptop...