You know how it is, you are on the phone explaining something to a user and you really, really, reaaaaaaaally would like to take over their screen. There are a lot of commercial products to set this up but I have found a free alternative: ShowMyPC.
The tool has a nice simple process for setting up the connection explained here. Some Virusscanners and Vista may give a security warning which can be ignored. The connection is setup via SSL (Port 443) so it should work for most users.
You can even set up your own SSH server if you are concerned with security,