How to lock a Windows Form

If you want to show the small window on top,
use the code

smallwindow.ShowDialog(); //where smallwindow is your smaller form

instead of

smallwindow.Show();

Try starting the second form as a child of the first:
Form2 form2 = new Form2(Form1); // or new Form2(this) if called from within Form1

And make Form2's constructor take a parameter that is the parent form (and then set its own Owner property).

Alternatively, set the Owner property directly: form2.Owner = form1;.

Child forms are always shown on top of their parent. They are also automatically closed when the parent is closed.