reinstalling XP

I had to reinstall XP again 2 days ago. what a carry on, that was the 4th time I've reinstalled it over the past 4yrs due to system upgrades.
got the message on the monitor "you have allready intalled this product 3 times, go to MS to confirm your licence"
I got a freephone number from the screen, so phoned them up.
the guy asked why I was reinstalling again, I told him, then he asked if I had another PC in the house, I told him yes, my wife has her own PC with her own version of XP.
then he started to give me a sequence of numbers, 42 of em. duly typed them in, and the software was activated.
what a rigmarole, they know youve got a licence, all they want is more dosh out of you to keep the MS empire happy. he must've thought I had 2 PC's working on the same sofware. If you have, you've got to get software for each PC you have