I would recommend logmein. With remote desktop, your friend will not be able to see what you are doing while you are working on the computer, which is not the greatest option. To configure it, you will need to set up port forwarding on your friend's router. Since your friend most likely does not have a static IP address, you will need to get a dynamic ip updater or else you will simply need to ask him to give you his public ip address when you want to connect, if it has changed since the last time.
VNC requires the same type of router configuration and has the same problems with dynamic ip address.
LoMeIn (there is a free version available) allows you to remote control another computer without configuring anything on the router, and without worrying about changing public ip addresses. It is easy to use and install. Go to www.logmein.com and get an account and install the software on your friends computer. The remote control is done through a web browser--simply log into your account. I've used logmein a lot--its simple and convenient.
