I found this rather usefull art. about IPSec and hope this will help someone!

Introduction

Everyone wants their server to be as secure as possible, but there are many different approaches that can be taken to accomplish this ends. Now Microsoft didn't invent IP Security (or IPSec for short), it was developed by them in conjunction with Cisco and the IETF, but Windows 2000 has a very robust implementation of it built in. The intent of IPSec is to help in creating secure connections between different machines, even when the software that's communicating has no knowledge of the encryption. IPSec can also be used to apply rules as to what kind of IP traffic a machine will accept, akin to a limited firewall - that part of IPSec is what this article deals with. I'm going to assume that if you're reading this, you already have Win2k installed and know how to get to the IPSec administration portion - if you don't, please check the additional resources section.

This came from

http://www.analogx.com/contents/articles/ipsec.htm

Play with IPSec to learn how it works.........Good-luck