VNC - XP Pro, still get "Cannot Connect to Server"

VNC - XP Pro, still get "Cannot Connect to Server"

Hi,

Having some trouble connecting to my XP Pro tower via VNC. I can connect via RDP just fine. Is there something I have to do in my router? Port forward or something? Can you walk me through it? Auto Discovery yields an RDP to this XP Pro tower, which is functional, but I need to walk my employees through training remotely via VNC remote shadowing, so I created a manual connection, grab the IP address of the XP Pro tower from whatismyipaddress dot com, which I know works because I have a camera system setup that I can view remotely, and I get a cannot connect to server error. I am sure it's something simple that I have to do on the machine, but need a little assistance. 

I have PocketWise Pro, love the product, just need to do this last thing and it will be completely worthwhile.

Thanks for your support.