Download, install and configure a VNC server application of your choice (RealVNC, UltraVNC, TinyVNC, etc.). I use UltraVNC but have also use RealVNC. Google will find the primary web pages. Configuration is fairly trivial. You may need to open Windows XP firewall (or any replacement firewall you might have installed). If you are going through any other firewalls, you'll need to open the appropriate inbound ports as well.
Review the documentation on the site for the application you select. Come back with additional questions. You do the work; we're here to help.