This was a SharePoint problem for us. If you go to your IIS on the server in question under application pools and find the site in question, go to Properties, then the Identity tab, to the place where you can change the user/password. We had recently changed the service account password, but did not know we needed to change it here as well. Once we did that things started working again.
This problem occurs if the Net Logon service on the domain controller does not start before the World Wide Web Publishing Service starts. See ME929100
for a workaround.
This issue occurs if the password of the Communicator Web Access service account has changed or has expired. Therefore, the Communicator Web Access service uses an incorrect password to try to start. See ME931507
to solve this problem.
From a newsgroup post: "I seem to have resolved the issue by recreating the application pool account (delete / recreate)".
See "Configuring and Troubleshooting Windows 2000 and Windows Server 2003 Certificate Services Web Enrollment" for details on this event.