In order to install a web application running only in 32-bit mode, I had to change the default IIS application pool settings to allow 32-bit applications. This worked fine for my application, however later I installed SharePoint 2010 on the same machine and when I opened the Central Administration I got the wonderful:
HTTP Error 503. The service is unavailable.
No IIS logs, no ULS logs, not much clues… The SharePoint Central Administration application pool was always shutting down for some reason.
It turns out that if Enable for 32-bit Applications is enabled SharePoint will not load and turn down the application pool. I changed this setting back to false and I was able to get to the Central Admin.
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Found this checklist to resolve this error in SharePointDiary.com -
Service Unavailable: HTTP Error 503. The service is unavailable in SharePoint