Okay, that's a lot easier then! Unless you have a static IP address assigned by your ISP then you'll want to set up dynamic DNS on your PC (see http://www.dyndns.com/). You'll also need to forward the necessary ports on your router (see http://www.realvnc.com/support/portforward.html).

The only issue is that the VNC protocol is unsecured - all traffic after the initial login is sent in the clear. To protect this you should use an encrypted tunnel, which is a little more complicated to setup, but I can write up some instructions if you want to do this.