Recently I changed my desktop from 32bit to 64bit Windows 7 and from there onwards I keep getting such issues. In such case when you run web page which gives following error.

Error : Could not load file or assembly 'XXX.XXXXXX' or one of its dependencies. An attempt was made to load a program with an incorrect format.

Error: Failed to complete setup of assembly (hr = 0x8007000b). Probing terminated.

When you enable fusion log to see assembly load trace then you can see following detailed error.

I found one workaround for this; you need to enable 32-bit application on application pool.

Here are the steps for workaround.

• Open IIS
• Go to Application Pool
  

• Select application pool in which your application is currently belongs too; right click on it and select Advance Settings.
  

• Advance Settings dialogue box.
  

• You can see Enable 32-Bit Application is set to False.
  

• Make it true and refresh your website page.
  

This should fix the issue, you can see your website up and running.