It depends on the order in which you installed the different versions of Windows.
If you installed XP first, to the first partition, Stephen's approach would be exactly what you would have to do.
BUT if you installed Win7 first, removing it might make XP unbootable. It would remove the boot.ini file.
You should probably make a full backup of the xp partition (win7 too if you want to keep anything from it), format the entire drive to wipe the whole thing clean, then put XP back on it in the right place...the first partition.
Make sure you have a working boot.ini file for booting xp on the first partition, though, or it will not boot. If you do a fresh install of XP you should have one. But if you mirror what you have now you might not.