1) You can enter Recovery console, usually admin password is not set in XP. When asked for admin password, just hit <ENTER>.
In cmd promt: type cd $ntservicepackuninstall$\spuninst

2) Can you log in to xp in safe mode(F?
If you can, then log in safe mode, go to section control panel --->add/remove softwares---->choose to remove XP SP2.
Or Start--->Run---> type %SystemRoot%\System32\restore\rstrui.exe

3) Log to xp save mode, run system restore.

Details: http://support.microsoft.com/default...&Product=winxp