The Open Command Execution within the registry can sometimes be changed or altered when a client has multiple web browsers installed on their machine.  This can cause applications that have default values to not work properly, such as outlook or excel not opening a web browser when you click on a link.  To fix this issue reset Internet Explorer to the default browser of the machine.
To do this open Internet Explorer and go to Tools > Internet Options > Programs (Tab) > and select “Make default” under “Default web browser”.

– Harry Caskey