If you get this error that says:
"An error has occurred while attempting to load the Crystal Reports runtime.
Either the Crystal Reports registry key permissions are insufficient or the Crystal Reports runtime is not installed correctly.
Please install the appropriate Crystal Reports redistributable (CRRedist*.msi) containing the correct version of the Crystal Reports runtime (x86, x64, or Itanium) required. Please go to http://www.businessobjects.com/support for more information. "
Open your IIS and click "Application Pools", select the application pool name that your application is using, then on the right side click "Advanced Settings", which will show you the advanced settings for your app pool. There set the "Enable 32-Bit Applications" to True. Click Ok, and now go check your page which uses crystal reports, it should work just fine.
This info was for 64-bits Windows 7 rocking Visual Studio 2005, may also work for few other microsoft operating systems and iis.