Unable to find BrowserApplication namespace/type

Hi,
I am using browser functionality by referring below link.
http://stackoverflow.com/questions/13621467/how-to-find-default-web-browser-using-c
I could not find the assembly in which BrowserApplication namespace/type exists. I am getting error on the below line.
BrowserApplication browser;
The error is "The type or namespace BrowserApplication does not exist". Please let me know the correct assembly to reference.