I can just see it now
Add/Remove Programs:
Word 2000
Excel 2000
Quake 3 Arena
Windows XP
Winamp
Heh, sorry, i guess that that really didnt answer your question.
Normaly, Windows upgrades are final, they basicly change most of the important system info, and short of windows xp backing up all of the win 98 files (which i highly doubt that it does) you are pretty much stuck with it unless you wanna re-format your HD
Hope this helps,
Mike