If you have some Macintosh icons that you wish you could use on Windows, then download v1.6 of the freeware Icon Editor IcoFX. This is one of my favourite pieces of freeware.
You can either load a .icns file directly, or extract the images from a ._* (Mac Binary) file. You can then either Save As a Windows or Macintosh icon, or use the toolbar shortcuts to generate your icon type of choice.
If you have a large number of icons, there is also a Batch Process option.