The latest version of NetMon is now available for download via the Connect site (Passport and registration required).  If you've ever used NetMon 2.0 (the version included with Windows Server 2003), you know that that it's a bit dated (to be kind).  NetMon 3.0 offers intelligent filtering, real-time capture and display, a completely new UI, and much more.  Highly recommended:

http://connect.microsoft.com

Click on Available Connections and the scroll down the page until you see "Network Monitor 3.0 beta".  Click on Apply to begin the registration process.