I have an illegal downloaded copy of XP, and have installed on 4 different PC's in the past 3 years.When it came to activation, I simply phoned the number shown on the screen, and said I had just purchasd the computer second hand.Each time I did this I was given a new code to enter, and everything has been fine since.Its been about 12 months since I last did an installation, and I hear the Microsoft are clamping down on counterfeit products, so I am not convinced it would work if I tried the same today.