XP generates a 5- digit code for the computer you install it on. The code is based on the hardware components in your machine. According to MAXIMUM PC (Nov 2001), "If you try to activate the same registration code with a different hardware configuration, the proverbial alarm bells sound, and the anti-piracy police come a-running to put the kibosh on your sneaky little operation." Their tests showed you may be able to change out up to 6 pieces of "monitored hardware" within a 6 month period without having to call Microsoft for permission to reactivate your OS. If you do change out more than the allowed hardware you have to call and get another activation code.