Since you have both in the same partition, just boot into Windows 2000.
You had to have installed W2K into a different folder then the WinNT folder.
You can just delete the WinNT folder.
If you want get the NT 4.0 off the boot display you have to edit the boot.ini file.
This is a hidden file on your C drive.
You have to use the Attrib command to remove the System and Hidden flags to edit the file.
You just remove the line that Shows Nt 4.0 and make sure that the default line is for Win 2K
If I were you, I would leave the boot.ini file alone! No sense in creating a problem. The computer will just not boot up if you select NT 4.0 as the boot system.