PC only text

Can someone write me the following scripts - So there is a link which only appears on a PC, another one which appears only on a Mac and another one which says "This is not available for (followed by the Operating System)."

Can you also please describe how it works. Thanks.

A little hard for us to do... you forgot the link/attachment.

Oh, sorry, it's http://trainweb.org/hsteu/example.zip