Here's a simple trick that I've used countless times for enabling remote desktop on an XP or 2003 machine remotely. Windows 2000 requires a service to be installed, so this won't help there.

Just connect to the remote machine with regedit (or use reg.exe from the command line if you want), browse to  HKLM\System\CurrentControlSet\Control\Terminal Server, and set or create fDenyTSConnections (a DWORD) to 0.