There are several things to think about when taking control of another computer, the Operating System running on it, the speed of your network connection and the tools you have at your disposal.
How To Take Control Of Another Computer
Operating System
There are 3 main choices of Operating system that the computer you want to take control of might be running, Windows, Mac OS X and Linux. Fortunately Mac OS X is based on BSD so the tools you would use to take control of it are the same as you would for Linux, simplifying things some what!
Network Speed
If you have a fast network it’s possible to use remote desktop tools, such as Windows Remote Desktop, Virtual Network Computing (VNC) or on a Mac or Linux server you can do X over SSH.
How To Take Control Of Another Computer
Windows
To enable Windows Remote Desktop, click the Start button, click All Programs or Programs, and then click Accessories.
Mac / Linux
How To Tunnel X Over SSH
If you want to know how to take control of another computer that has X Windows on it, e.g. a Mac or a Linux machine, but even really old Solaris or other styles of Unix, then you need to make sure the machine you want to take control of has in it’s sshd_config file
X11Forwarding yes
You can then connect to the remote machine with
ssh -X hostname
Then any GUI applications you run via the command line will magically appear locally on your machine.