import javax.swing.*;
//import all your other classes as usuall

class MyJFrame extends JFrame  
{
       public static void main(String[] args)
       {
           try
           {          
              UIManager.setLookAndFeel(
                      UIManager.getSystemLookAndFeelClassName());
             //This will set the look and feel to the platform
             //So it will look like a windows app on windows, a gtk app on
             //linux etc. 
            } catch (Exception e){}

        //now you can do all you other code, like create the instance of 
        //MyJFrame and they will all look like windows forms. 
       }
}