Platform: Windows 2000, 2003, XP

If you are having a problem seeing your machine dynamically register itself in DNS, make sure you are running the DHCP client services. Logic would dictate that you don't need to run this if using a statically assigned address (which is what I do on all of our servers), but the DHCP client service is what takes your IP and registers it in DNS. Re-enable the DHCP Client service to automatic and reboot (just starting it didn't seem to do it for me), and you'll see it in your DDNS server again.