How would I setup Windows Server 2008 as NAT router with only one nic? I only have access to a server with one nic and I can't add a nic to the server.
Short answer - you can't. In order to route something, there needs to be connections to two different networks.
Long answer, maybe you could if the one nic was assigned ip addresses on two vlans on the same switch and routed between them. I have not tried this, and I am not sure it would work. It would be much easier to add another nic. I am thinking it is a hosted solution you are working with, and if you cannot add a nic, you probably do not have access to the switch to implement the vlans either.
I've wrote simple 11 steps to setup VPN + NAT with single NIC here : https://plus.google.com/u/0/108197739169919071168/posts/bnDbKbrFPq6
here is steps :
( I selected 192.168.100.x when my client ip is 192.168.1.x )
VPN Server+NAT is now must work for you
I did it myself and I'm posting this post while I'm connected to that server!