Right follow these instructions. First off you need to have your Xbox connected to your network. Most XBMC users will have done that, if not don't bother trying this.
On XBMC select Settings then Network and write down the IP address of your Xbox, go down to Servers and enable web server. Leave the password blank. Now the good bit... Open the browser on your PC and go here :http://xbox_ip_address
And there you have it, a mini VNC connection;-)