Recently, one of my teammates installed ArcGis Server 9.3 on our fresh SharePoint 2010 box, causing all sites to respond with 503 Service Unavailable. A quick inspection showed that the application pools have stopped themselves, and the Windows application event log was spammed with:

Event ID: 2282
Source: Microsoft-Windows-IIS-W3SVC-WP
Description:
The Module DLL C:\Program Files\Common Files\Microsoft Shared\Web Server Extentions\14\isapi\owssvr.dll' could not be loaded due to a configuration problem. The current configuration only supports loading images built for a x86 processor architecture. The data field contains the error number...

After some investigation, I found out that the installation switched all of the application pools into 32-bit mode, causing SharePoint's x64 dlls to fail to load.
The solution I found - go over every pool in the IIS manager under Application pools, open advanced settings and set Enable 32-Bit Application to False.
Also, make sure you click Set Application Pool Defaults and change the settings there, for your future pools.