You will have to format your hard drive and install a fresh copy of 2000.
What you are suggesting is a waste of time though. A complete waste of time. XP IS 2000, but with more features. If you want to do it, at least recognize that you'll be losing some features that you might want such as remote desktop, support for Hyperthreading processors, improved networking, etc
If the Win2000 cd is bootable, just boot off of it. Hopefully it's like XP and will allow you to delete a partition. Delete the partion, or format, or whatever it lets you do. Then create another partition, or if you just formated, go ahead and install.
Alternatively, boot into dos, most likely with a win98 boot disk, and type format c:
Then boot off the 2000 cd and install.
There are alternative ways to install, by copying the contents of the cd to your hard drive, but the above should get you going just fine.
Don't forget to backup important items, including fonts.