There is only one way to "uninstall" XP.............format the drive which
will erase everything on that drive.
You can however do a "repair" installation which will overwrite your
ecxisting installation with all new files directly from the installation CD.
This however also requires you to download all of the updates that have been
released since your CD was issued.
Then there is the problem of SP2...if you have updated to SP2 you will not
be able to repair with a version of XP prior to XP2 ...you will need to
slipstream the SP2 update package into your XP CD onto a new CD and use that
to do the repair installation.
here is a website that details those events
http://michaelstevenstech.com/XPrepairinstall.htm
peter