Came across this nasty error the other day whilst trying to get an IIS app running on my Visat 64bit PC:

Error: Is not a valid Win32 application. (Exception from HRESULT: 0x800700C1)

Solution:

1. Inside IIS (7) click on Application Pools (left hand side under {machine_name}).
2. Find the Application Pool that is configured for your web application in the list.
3. Select it, then click Advanced Settings on the right.

4. Second setting in the list: Enable 32-Bit Applications - must be set to True.

But some time ,not the case,Windows XP ,IIS5.1,thorw it .

I think  something wrong with the iis may be crashed.

something may be like this.

aspnet_regiis -i  and restart.