HTTP Error 503. The service is unavailable.

Okay. Took me some time to fix this. Usually I don't post things like this but after spending some time on searching for an answer (which there is none at time of writing) and after experimenting here and there I finally found out what went wrong and how to go around it.

I updated a Window patch on my My Win 7 Pro updated itself few days ago and that kinda did something to my ASP.NET 4.0 - IIS 7 combo. It cracked it. I am stunned to be presented with this screen

Okey, you are probably here for answers so... long story cut short... fix it by going to your local IIS Application Pools -> Choose your Pool (ASP.NET v4.0) -> Advanced Settings -> Process Model -> Identity

I changed it to "Local System" to get it working. I am not advising you to do this cause that's a lot of power given to the Pool. The recent reason this error surfaced is because the patch somehow screwed up the access rights of the original ApplicationPoolIdentity. Quick fix just to get it working again. To really fix it, give the rights back to the ApplicationPoolIdentity.

This fix only works for access rights issues and does not work for all types of "HTTP Error 503. The service is unavailable." that might creep up in time.