If you recently downloaded the latest ajax toolkit (1.0.10606.0) from Microsoft, you may receive the below error:

 System.Configuration.ConfigurationErrorsException: Could not load file or assembly VsWebSite.Interop

 I was able to eliminate the error by removing the below line from the web.config:

<add assembly="VsWebSite.Interop, Version=8.0.0.0, Culture=neutral, PublicKeyToken=B03F5F7F11D50A3A"/></assemblies>