Remote Desktop if the computers are running in you LAN. You already have it if you are running windows 2000 server or better. Of course if the target computer is on the LAN you could just walk over to the computer. :)

Across the internet or WAN I don't think Remote Desktop works, you need a 3rd party program like you've mentioned.

I have tried doing something like this using Net-meeting by Microsoft for my home computer. My wife calls me a lot when she can't get something to work. Unfortunatle Netmeeting never works for me across the internet. Probably a combination of going through my work firewall which I can't configure.

-Steve G.