Uninstall is only an option if you upgrade initially (and even then only
under specific conditions). Generally, removal from the old involves
formatting or deleting the existing system partition. Only retail versions
of WinXP are movable from one system to another. Preinstalled or OEM
versions are generally permanently tied to the system that they were
installed and activated on.
Boot with the Windows XP CD in the CDROM drive then work your way through
the menus until you come to an option to format your hard drive. That will
remove it.
Tricia