How do I switch between 32-bit and 64-bit versions of ASP.NET on IIS 6.0?


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On a machine running a 64-bit operating system, it is possible to run the 32-bit version of ASP.NET 1.1, the 32-bit version of ASP.NET 2.0, or the 64-bit version of ASP.NET 2.0. Issues may arise when both versions are installed, but incorrectly configured. The following error message may display:

Service Unavailable

The following error messages may be found in the Application log in the Event Viewer:

Could not load all ISAPI filters for site/service. Therefore startup aborted.

ISAPI Filter ‘C:\WINDOWS\Microsoft.NET\Framework\v2.0.50727\aspnet_filter.dll’ could not be loaded due to a configuration problem. The current configuration only supports loading images built for a AMD64 processor architecture. The data field contains the error number.
To learn more about this issue, including how to troubleshooting this kind of processor architecture mismatch error, refer to the following URL:



It is possible to run ASP.NET 1.1 and ASP.NET 2.0 concurrently, but only with IIS in 32-bit mode.

Microsoft has provided a Knowledge Base solution explaining how this can be configured:

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: