A customer recently had a intermittent problem with a Windows 2003 server.  After a reboot, the server would appear to be running, but was not available over the network.  In addition, the IPSEC service didn’t start properly and was in “blocked” mode.  The exact error messages we received were:

The IPSec driver has entered Block mode. IPSec will discard all inbound and outbound TCP/IP network traffic that is not permitted by boot-time IPSec Policy exemptions. User Action: To restore full unsecured TCP/IP connectivity, disable the IPSec services, and then restart the computer.  For detailed troubleshooting information, review the events in the Security event log.

Followed by:

The IPSEC Services service terminated with the following error:
Only one usage of each socket address (protocol/network address/port) is normally permitted.

Here’s the fix: http://support.microsoft.com/kb/956189