The so called SmartScreen Filter has been added to Internet Explorer 8 by Microsoft. The filter has been designed to warn the user in case unsafe websites are accessed in the web browser. Some Internet Explorer users on the other hand might prefer to deactivate it. Either because they are using a security software that checks the websites for them, like Web of Trust for instance or a security software that integrates in the web browser and checks the accessed websites, or because they do not want to transmit information about the visited websites to Microsoft.