(14 November 2012)
The web server on Server 2003 appears to run as the hidden special user
Network Service. Anything that the web server needs to write to needs to have permissions set with that in mind.
If you are running ASP Application pools, you need to double-check the user process that the Application Pool is running as, and ensure that that user has the appropriate permissions. (In my case it was still
Network Service, but sometimes it is different.)