We had a windows 2003 Server that after a reboot we could not login to the Domain or get to the internet.  Basically the NIC seemed to be non-operational.  We could not Ping the server but we could ping the local loop and local IP.  So I started troubleshooting routing and that led nowhere quick.

After looking at some issues with IPSec Service not starting I found the following MS KB.  http://support.microsoft.com/kb/912023

I ran through the steps and noticed that the entire IPSec Registry Key was missing so I knew we were on the right track.  Before running the command to recreate the Key and values I manually created the IPSec Key and Policy sub-key.  Then ran the following command:

regsvr32 polstore.dll

And that did the trick!  Server rebooted and came up normal with no issues.