|Why would you want to do that?|
You'll have to wipe the hard drive & start from scratch. But before you do it, make sure to back up all your important files, pictures, word docs, etc. Then make sure you have all the necessary 2000 compatible device drivers (chipset, video, sound, network, etc). And if your 2000 disc doesn't have the latest SP, you might wanna download it in advance:
Windows 2000 Service Pack 4
Once you have all that, just boot off the 2000 CD (make sure to "press any key..." when prompted), then follow the installation instructions.