If you are using a 64Bit Operating System you will need to allow 32 Bit application to run from your IIS.
To do so:
1. Go to the IIS Manager.
2. Open "Application Settings".
3. Select the "Application Pools" item on the tree on the left.
4. Right click on the "Classic .NET AppPool" from the list.
5. Select "Advanced Settings".
6. Change the "Enable 32-Bit Applications" setting to "True" as shown in the screenshot below.