You're a bit confused on the client-server concept. If you are using a Redhat box to connect to a XP/7 box, then the Redhat box acts as the 'client' on this case, and the XP/7 acts as the 'server'.
Of course, your Redhat box can still be a web 'server', etc.
With that, you need to install a VNC 'server' on your XP/7 machine. You can use UltraVNC
as mentioned by previously.
Let us know once done so we can proceed with the next.