I have a Windows 2003 standard edition server behind a standard adsl modem/router.
Is it possible to set up the server as a VPN server without buying a second Nic card and directly connecting this to the internet?
There are a number of ways that you can configure your router to allow VPN access:
You'll have to configure your router to forward the ports necessary for your VPN (for example PPTP uses TCP 1723, GRE 47) to the Server.
I (very) strongly recommend you replace your router by one that will act as a VPN server. I use the Draytek 2820 routers (over a hundred of them!) and they work very well.
The point of using the router to do the VPN is that it's simple and easy to maintain and takes the load off your server (though admittedly a few VPNs won't load the server too much). If you ever want and LAN to LAN VPNs, to link a remore network rather than a single user, the router will do these too.
JR
Both responses from @renniej and @sam-cogan are spot on. I have just replaced my aged Draytek 2600 with a 2820 and it's very good. The 2600 are perfectly adequate for VPN if you want to go down the second hand-option.
I use my Drayteks to also do site-to-site VPN to a box running pfSense which I have performing IPSec termination for site-to-site VPN but also PPTP pass-through to a Windows Server 2003 RRAS VPN Server for end-user connections.