We were recently trying to get VPN access to an SBS 2003 standard install. Everything we tried just didn't work. We ran and re-ran the wizards, checked that the right ports on the hardware firewall were forwarded but still no luck. Typically, we would get the message that the VPN was connecting but during authentication it would simply timeout and we would receive a message that the VPN had been disconnected.
Turns out that the problem lay with the hardware firewall. What finally ended up resolving the problem was a simple upgrade of the firewall firmware. Once completed the VPN worked a treat. Initially you just never stop and consider that the hardware firewall ( external to SBS2003 ) could be the issue. It works and has always worked so why should it be a problem? Well, in this case it certainly was the problem.
Another handy tip we'd give is you is to always backup the configuration of the hardware firewall before you upgrade the firmware. Over time a lot of changes can be made to a hardware firewall that are note always documentated.