-First check if your cables are connected correctly. Also the message 'medium disabled' in "ipconfig /all" can point in this direction.

-DHCP is not enabled. If DHCP is not enabled, do the following (for Win XP): Start - (settings) - Control Panel - Network Connections - Right-click the LAN-connection - Properties - Select Internet connection TCP/IP and choose Properties - Tab 'General' - "Obtain an IP-address automatically - Obtain DNS server address automatically" (enable both) - Choose OK and close all windows.

-An error 800 can also occur if the IP address for the VPN connection is not set correctly.

-Other possible problems are caused by firewalls (possibly incorporated in anti-virus software) and/or routers.

Try to set up the VPN connection with the firewall switched off.
The latest version of Norton antivirus possible contains a bug which blocks VPN connections. A possible temporary workaround is to switch off the security for internet worms. Norton recommends an update of the program.

Concerning the router, check if the "VPN passthrough" option has been enabled. The VPN-passthrough can mostly be enabled by means of a webinterface (see manual). To do so, surf to the gateway-address of your PC. The gateway-address can -in a WinXP- be found via Start - Run - cmd - ipconfig /all.

-Disabled network cards can generate this error as well. You can check this with a right-click on My Computer - Properties - Hardware - Device Manager. A red cross or an exclamation mark indicate that there is a problem. Enabling the card is mostly sufficient.

-Exceptionally the cause is the physical address of your ethernet card (in this case the SiS 900-Based PCI Fast Ethernet Adapter). Sometimes ipconfig /all or winipcfg shows the address 00-00-00-00-00-00 and this causes trouble to set up a VPN connection and more specifically to reach the helpdesk without a VPN in the homes. A common LAN-connection for its part works fine.

Source: ugent.be