Uninstalling an operating system basically works by deleting all the disk partitions which are in use by the operating system that you want to delete.
So in theory it should be feasible to boot up the machine from the Windows XP installation CD (or does it come on a DVD?), launch fdisk and remove all existing disk partitions.
You might also boot up the machine from the Linux Mint live and installation system which you used to install Linux Mint on the machine, launch GParted from the Mint Menu and delete each partition on the harddisk.
Endless legions of bigoted, brutal, bloodthirsty Orcs from the dark land of Mordor are bringing desolation, despair and death to the poor inhabitants of the Middle East.