Did you panic when you first saw
“Service Unavailable”
A recently released update KB 973917 from Microsoft that relates to the “Extended Protection for Authentication” Architecture first added in KB 968389 is causing issues with some Win2003SP2 servers. If the installation of Service Pack 2 on the server did not complete entirely, then some IIS related DLL files were never updated properly. These older DLL’s were never an issue until now, and there was no obvious way of knowing that the SP2 install didn’t completely successfully. Fortunately a resolution has been found for the application pool stopping… reinstall Service Pack 2.
KB relating to incomplete SP2 install
http://support.microsoft.com/?kbid=2009746
KB973917
http://support.microsoft.com/kb/973917
Extended Protection for Authentication
http://support.microsoft.com/kb/968389