I just used TeamViewer to allow a remote user in to perform an upgrade on a server.
Similar to LogMeIn, GoToMyPc and FogBugz CoPilot.
The nice thing is that it’s free for non-commercial use and you can download and run the client on the PC without doing a full install. Once you stop it running it has gone, so no need to worry about someone reconnecting later on. It gives you an identification code and password which you give out to the external user (over the phone or whatever) who can then connect and take control of the machine.
As it’s running over standard ports there’s no firewall configuration to get in the way, so this will be ideal the next time a friend or family member wants me to connect remotely and fix their PC.