If upgraded, then I'd just start again. If you installed into different folder (which by default you would - 2k defaults to \winnt, XP to \windows), then you should now have a dual boot menu when you start up. You can remove 2k in this scenario by editing boot.ini in the root of C: to remove the 2k entry and more importantly, you can delete the \winnt folder. However, \program files will still contain apps you installed under 2k (though they won't be available in xp) - so you should manually delete these (if you want then to run in xp, will need to reinstall).
Question - how large is the hard drive? Drives are not expensive - might be worth investing in a new one.