OK, I took a chance on that ebay WIN XP # and it worked! I just finished building a new AMD 1800+ system and decided to try and use that #. I do own a retail version of WIN XP PRO and used that disc. The # worked fine and so far, I haven't seen the "you have 30 days to register this software" message like with my retail version. So far so good, i'm loving it. Ya it's nice to have a retail version of XP, but looks like this # could've saved me a lot of money. Oh well, happy chatting!