Remote login and controling another computer

Im after software/technology that will allow me to log in, see and controle another computer (mac or windows) from my current computer over a network.

How can I do this? is there a commonly used protocal for this? can you recommend any programs for mac (and windows as i would like to connect this this system) that will allow me to do this?

What port does VNC use so that I can set up port forwarding on my router?

Thanks

Microsoft do a remote desktop computer for osx use it myself and its quite good.

its on mactopia

I have had a fiddle since my first post and it seems cool.

Can aonyone recommend me a very small lightwaight VNC server that I can put on a windows box? Going to use it as a server but want to controle it from my mac.

I always use UltraVNC on my Windows machines.