|Yahoo is not an Email programme|, Yahoo is an Emaail provider.
To set a default Email client you need to isntall one, like Outlook or Outlook Express or many of the free Email clients available. None of them are tied to any particular Email provider.
If you do not have an Email client installed then you must be accessing Yahoo via their web interface in which case your Email programme is your web browser. You can configure your browser to use Yahoo as the default Email provider