Why don't you ask on the Smoothwall forums to see what recommendations the people there can give you? The forum are very active, and there's always somebody who will be able to help you.
Although smoothwall (and all other Firewalls like IPCOP etc) do have VPN features builtin, this VPN config is typically used to connect two or more branches.
Microsoft RRAS (VPN) solution is a total different one. You can however configure a standard incoming PPTP connection on your Windows server and use port forwarding on Smoothwall to send VPN traffic to Windows box.