In your case, I think you should have the Execute method as a static method.
In your class, hold the object of your form privately, so that the outside
world won't be able to create the form object 'accidentally'
If the world can create a System.Windows.Form, then it will be able to call
ShowDialog.

or you can define your methods publically and call do the checks and call
your execute method

public new DialogResult ShowDialog()
{
   return this.Execute();
}

I assume Execute() method returns a DialogResult

HTH,

--Saurabh