There are many possible causes. Make sure both computers are using different IP range. Or this search result may help,

VPN Issues Cannot access remote computers via VPN because of NIS · Cannot access resources after establishing the VPN because of the same IP ...
     http://www.chicagotech.net/vpn.htm

Bob Lin, MS-MVP, MCSE & CNE
Networking, Internet, Routing, VPN Troubleshooting on http://www.ChicagoTech.net 
How to Setup Windows, Network, VPN & Remote Access on http://www.HowToNetworking.com 
 OK. I have got the VPN working, (more luck i think), but can not get any
 further. I can not ping, or see the computers the other end. I would like to
 use sonthing like activesink to keep my files upto date on my laptop and home
 computer.

 Any idears please.

 Dave