Started the day with a client email telling me the site was unavailable, as in

Service Unavailable

unavailable.

Seems the network admin applied the latest Windows 2003 Security Update on the machine last night, which, little did I anticipate, change the IIS6 process identity.  From a google thread:

The process identity for IIS6 changed from Local System to Network Service for security reasons since it is bad security practice to run as "Local System" -- one successful buffer-overrun means your box is hacked (like Code Red/Nimda).

Fortunately I found that within a minute of two of googling on ((security and update) and "service unavailable".)  Another instance where library school paid off.  Hm, interesting topic for another post....

Changing the site's application pool from the app pool with the local user to the default application pool did the trick.  I encountered and fixed other permissions-related errors as a result of the security changes, but it was an annoyance that with no warning sites are crashing due to a windows security update.  And now, I guess, local accounts are a bad thing.