ok then...
visit
http://www.microsoft.com/windows/xp for the official info...
XP stands for eX-Paper clip, as far as i know, it's named after the latest version of MS Office which is apparently so simple you don't need Clip-It, your paper clip assistant...
basically, the whole of the system has been written from scratch which makes it more stable. unlike 98 which (i think) was written around the 95 core...