Provided you don't need the title bar, or the system buttons, you can make any form (or any control) any shape you like by setting it's Region property:
GraphicsPath path = new GraphicsPath();
Rectangle rect = Rectangle(0, 0, 200, 200);;
Region = new Region(path);
This sets the form to a small circle.
If you do this, you will be responsible for making sure that your controls are visible, you will need to do border effects yourself,and ensure there is some way for the user to close / minimize it.