Question : VPN help:  error 721, the remote computer did not respond

I am trying to connect using VPN to a SBS 2003.  I'v enabled port 1723 for VPN. I got the about error message. I'c been searching for solutions and haven't found one.  Help me please!!!!

Client : window xp
Host: SBS 2003 with SonicWall router

Answer : VPN help:  error 721, the remote computer did not respond

Is 2003 sbs the VPN endpoint?  Is that the device that authenticates?  Is that setup correctly ?  Possibly the router is configured correctly but there is a problem in another area?