Have a look here http://www.tightvnc.com/,
vnc (Virtual Network Connection) is software for remote controlling a PC, using an IP address, tightvnc is a free version, from a different company.
Just for completion, if your remote desktop happens to be a Windows 2000 server then you can install Terminal Services on it then run it in remote administration mode. I think Mattie has the best solution, though.
Once you run VNC Server on a machine it will accept connections and relay through the keyboard, mouse and display to the VNC Viewer. Each can run on any OS, but obviously if you use VNC Viewer to remotely use a windows machine, the display that you'll see will be that Windows machine. This is not the place to explain how to set up and run VNC though. If you need more help aside from the manual itself, email me at : [email protected]