I know what you are talking about, but for some reason its not given me the option in this project.. ill try another soon or start over ect..!
The option to set an assembly as being processor-specific is buried in the VS UI, it isn't immediately obvious.
You can either change it via a hidden menu on the Solution Configuration window, or open Project Properties > Build > Platform Target > "Any CPU" | x64 | x86.
...though I don't see how running an instance of the WebBrowser control would cause the Default Programs option for IE to change. Something doesn't sound right.