Another dumb mistake:  installed VMware Server 1.8 on an Exchange 2007 SP1 server with the client access role installed.  Instead of choosing the “custom” install option, as I usually do, I was in a hurry and chose “complete”.  Halfway through the install, I remembered why I never blindly choose “complete”; VMware Server Management Interface.  VMSI is a web-based management interface for VMware Server that when installed changes the preferences for IIS by enabling 32-bit ASP on 64-bit Windows.  This breaks OWA (64-bit ASP) because IIS 6.0 does not support running 32-bit mode and 64-bit mode at the same time on 64-bit Windows.

Here’s the fix:

Disable 32-bit mode

cscript %SYSTEMDRIVE%\inetpub\adminscripts\adsutil.vbs SET W3SVC/AppPools/Enable32bitAppOnWin64 0

Register 64-bit ASP

%SYSTEMROOT%\Microsoft.NET\Framework64\v2.0.50727\aspnet_regiis.exe -i

Thanks to:

How to switch between the 32-bit versions of ASP.NET 1.1 and the 64-bit version of ASP.NET 2.0 on a 64-bit version of Windows

HOWTO: DIAGNOSE ONE CAUSE OF W3SVC FAILING TO START WITH WIN32 ERROR 193 ON 64BIT WINDOWS