Software

I recently purchased a HP iPAQ business navigator.
I have successfully set up a VPN connection my office PC, when I make the connection I can see myself connected on the incoming connections screen.
However I cannot acces any files on the PC and consistently get the error message "network resource cannot be found or you do not have permission to access the network"
I have done the port forwarding for PPTP protocol through my router, also GRE forwarding.
I have tried turning off the firewall and my AV programme but still no success...any ideas please?
PS when I connect via my own wireless router I have unlimited network access but I need to do this when I am out as well