VPN working but can't remote desktop

My first post here, and im not sure which forum to post in for this one!

I am trying to remote desktop my remote computer via VPN between 2 draytek 2600G (both o/s xp professional sp1) - VPN is set up and running well and I have managed to do this to another site successfully and remote desktop.

With this one though I can ping the remote router, change settings on their router if I want, but i cannot ping the ip the router gave to the computer.

I am fairly new at this and could be missing something really obvious, I have tried running the network wizard on both computers but they are not seeing each other even though the VPN is there - as I said i am new at this and I have probably missed a crucial step!!!!

Any help would be welcome

Thanks