windows_icon .. how to?

[calmar <mac calmar ws>]

Hi, actually my last attempt was this:

   window = gtk_window_new (GTK_WINDOW_TOPLEVEL);
   iconw = gtk_image_new_from_file ("icon.xpm");
   pixbuf = gtk_image_get_pixbuf( GTK_IMAGE(iconw) );
   gtk_window_set__icon (GTK_WINDOW (window),GDK_PIXBUF(pixbuf));

The windows still have the same standart 'gtk' icon..
I guess the 'icon.xpm' file should be ok, (i tried with 16x16 and 32x32).
Where is the mistake (probaly big one) by my site .. how to setup the 
icon for the main window?

Great Thanks
calmar