IPSec
IPSec - it's a W2K paper but I can't find one on W2K3
More on IPSec...
Step-by-Step Guide to Internet Protocol Security (IPSec) ...recommended
Exploring Peer-to-Peer IPSec in Windows 2000
Using IPSec to Lock Down a Server...wasn't that guy in Zoolander? ;)
HOW TO: Use IPSec Monitor in Windows Server 2003