Hi,
I would highly recommend getting hold of Norton Partion Magic. It make the job very easy. Yiu install it onto your existing XP partion, then tell it to make a new partion for Windows 2K. Then install windows 2k, preferably from a boot CD. During setup it will ask you what partion you would like to install on, select the new one you have created with partion magic and hey presto, a dual boot system. As Dan said before you need a boot Manager, this is easly solved, windows on boot will ask you to select either 2k or xp, there you go, a dual boot system..
Rob