IIS 7.0 / 7.5 : How to switch between the 32-bit versions and the 64-bit version of ASP.NET 2.0 on a 64-bit version of Windows

Basically, go into your IIS 7 manager console, find the application pool your app is running in, right-click on it, go to Advanced Settings, and change the Enable 32-bit Applications setting to true.
Now restart IIS.