I've just reinstalled XP Home on my PC after replacing the mobo, CPU, case and psu and in the past have replaced the HDD and graphics card. So my PC is practically all new and XP activation wouldn't work. I was semi dreading having to ring Microsoft as I expected to have hassles about it not being the same PC etc etc. WOW - to my suprise the procedure was painless and suprisingly fast. No waiting for a customer service representative, no explaining anything, just given a new ID number there and then! As it happens it is my copy of XP Home but it could easily have not been!