[2008] Can't continue before window's closed

I'm trying to figure out what the code to do this is:
I click register, regform.show happends, then I want the user to finish with that form either finish it or cancel it before going back to main form. So what i'm trying to say is I don't want user to be able to access main form, while reg form is running.

Can someone help me with this? :) thanks.

Show that form using the ShowDialog method rather than the Show method.