Error 800: Unable to establish the VPN connection. The VPN server may be un-reachable, or security parameters may not be configured properly for this connection.
1) if you have firewall, open TCP Port 1723, IP Protocol 47 (GRE).
2) make sure you can reach the VPN server by using ping. Sometimes, poor connection can cause this issue too.
3) You may need to updated firmware on a router or firewall if other OS (win9x/nt/me/w2k) works except XP.
4) The VPN server may not be able to get IP from DHCP for the VPN client. So, you may want to re-configure VPN host networking settings. For XP pro VPN host, go to the Properties of the VPN>Network, check Specify TCP/IP address and Allow calling computer to specify its own IP address, and uncheck Assign TCP/IP addresses automatically using DHCP.
5) Make sure no other secure software blocks your access, for example, if you use Norton secure software, you may need to add the remote client's IP so that the client can access.
6) If your VPN running on a Windows RRAS with NAT enabled, you may want to check the NAT settings.
7) If you can establish the VPN from the desktop at home but not from the laptop. Make sure no security software like Microsoft OneCare software that blocks the GRE.