if you buy or somewhat get winXP, consider *tuning* it to your specific needs. with nlite
http://www.nliteos.com/you can remove unnecessary stuff like ie, media player, some services, unnecessary drives, codecs ... I made myself a XP light with firefox+thunderbird, VLC, removed windows internal cd burning and some stuff ... it works like magic !
it is totally legal as far as I know. I guess they get some help from MSDN as well.
