Upon inserting the Win 2000 install disc, you will have to do a complete format of the existing drive and OS, and then do clean install of Win2000 to the now bare drive. You will lose ALL INFORMATION on the drive, EVERYTHING, when you do this, so be sure to back up any data you do not want to lose.
Well, you'll need to set your computer to boot from the CD first, then put your Win2K CD in and run it (If you have any important files on the drive back them up before you install 2k). Then, run through the Windows 2000 setup and there you go, it's installed. You do know Windows 2000 is old and no longer supported by Microsoft, right? I repeat, back up files of any importance to you before you go and install 2000.