VNC Has a start server/stop server in the start menu, just have to make sure that when you are installing the application select it so that it doesnt run on windows startup, unless you need it to, then they can click start server when you need to access it and you type in their public IP address on your client.
Also you will have to open up a few ports on the firewall.
or you could use something like teamviewer which is web based and less hassle I guess?