VS 2010 Allow No "activity" Outside Of A Form?

I was wondering how you would go about making it so that, whenever you have a form open, or showing, that it won't allow any other clicks outside of the form content. Kind of like when a critical windows error shows up, and you can't do anything on your computer, until you press the "Ok" button to get rid of that popup message first.

How would you do that with a form?

Let's call it Form 1 and form 2 for example. I have form 1 as a default for when the application starts, but when I want form 2 to show, I want the user to have to deal with that form first before they can do anything else on their computer.