RealVNC

Enable RealVNC Server under Windows XP

Go to the control panel (Start->Settings->Control Panel) Select "Windows Firewall"

Select the "Exceptions" tab and you'll see a list of programs.

RealVNC should be one of those listed and will have a empty square to the left. Click inside the square to place a check mark.

Select OK.

If there are no other firewall's and if probably configured RealVNC server should be operational.