WindowsXP is NT 5.1, Windows2K is NT 5.0.
You can't install Windows over a previous version.
You should install the server off XP.
Boot the computer with a bootable floppy disk or with W2K server installation CD.
You will be able to install the 2K server but consider this: W2K will overwrite the NT Boot Loader of WindowsXP (5.0 over 5.1), you might not be able to boot to XP again until you restore the overwritten boot sector with a repair installation or through XP recovery console.