Notice that you might void your Windows XP license by running a VNC server on XP:
“Except as otherwise permitted by the NetMeeting, Remote Assistance, and Remote Desktop features described below, you may not use the Product to permit any Device to use, access, display or run other executable software residing on the Workstation Computer, nor may you permit any Device to use, access, display, or run the Product or Product’s user interface, unless the Device has a separate license for the Product.”
(from: Microsoft Windows XP Pro EULA)