yes you can. i just talked to microsoft about that the other day.
i was kind of worried about using my dads old xp disc since his old computer crashed, but they said as long as it is on only 1 pc at a time, it's fine.
you may want to do the phone in regristration though, just in case there is a problem, you can explain it to them and they can make a new product key for you if they have to