|There are a number of ways this can be done.|
Namely RealVNC, Remote Desktop or Remote Assistance.
Remote assistance could be the easiest way as it doesn't require any ports to be forwarded, you simply log into MSN messenger and get your collegue to do the same, then get your collegue to request remote assistance and you accept it.
The other two rely on ports being forwarded, useful if you want to remotely access a PC regulaly but one off it could be too much hassle.
For RealVNC download the free VNC Server onto the PC you want to connect to, set it up, configure it with a password then forward port 5900 from the router to the localmachine's IP. It can be connected to via the WAN IP address (which can be found by googling 'what is my IP' and sticking this into VNC Viewer which needs to be installed on the client.
For remote desktop it is built into Windows so you load up control panel, goto system, remote and tick allow incoming remote connections to this PC. Port 3389 needs to be forwarded in the same way as with VNC. This also can be accessed using the WAN IP, but rather than VNC Viewer, goto start, run, mstsc, hit enter then enter the IP in and it will prompt you for the password to login to the clients local Windows account, make sure this account is password protected else it wont allow a login.
hope this helps.
x2 3800 @ 2750
2 x 256mb pc3200 @ 229