Recently I was (re)setting up IIS from scratch on my development machine and kept getting HTTP Error 503 whenever I would try to load up a local web page.
For reference, in my case it's IIS version 8.5, running on Windows 8.1 Enterprise 64-Bit, but that really shouldn't matter.
Looking in the Windows Event Viewer shows:
Before losing any more hair, and/or trying anything else, give this a try, it worked for me:
Go to the Application Pool ...
Advanced Settings ...
Process Model ...
Identity ...
Set it to "LocalSystem"
You may or may not need to Restart the Application pool/Server afterward, but it's always a good idea anyway.
Enjoy!
Great Solved my problem