I've just recently converted from Win2000 to WinXP.
Decided to go for the HomeXP upgrade option because of the vast difference in price between the upgrade and full versions.
Got home with the CD and thought I'd see if I could boot from the CD rather than run the upgrade from the Win2000 desktop .. was surprised to find I could boot from the CD.
Backed up everything I wanted saving from my Win2000 setup and rebooted onto the WinXP CD .. chose to do a format from the options and waited for the installation to come up with an error message telling me I had no qualifying Windows OS .. installed with no problems through to completion.
Are good old Microsoft ripping us off in getting us to buy the full version of WinXP at twice the price of the upgrade version, when the CD from the upgrade version does the full upgrade with no problems.