I\'m currently working on a site which displays map data in Google Maps, but also has a link to display it in Google Earth.
To get view this data in Google Earth, a user
No, you can't force the users machine to launch an application like that. However, a user can set this up themselves if they wish in certain operating systems with certain browsers.
For example, in Windows I have Google Earth associated with the kml and kmz file extensions. I browse using Google Chrome. When I click a link to a kml/kmz file it is downloaded as per normal. In the download bar I can then choose to "Always open files of this type". Thus, when ever I click a link to a kml/kmz file it is automatically opened in Google Earth.
If a site could 'force' this behaviour not only would it be immensely annoying it would also be a massive security risk.
Also, why not just use the web based version of Google Earth? It is pretty simple to intergate it directly using the Google Maps Api.
i.e:
http://earth-api-samples.googlecode.com/svn/trunk/demos/mapsapi/index.html
http://earth-api-samples.googlecode.com/svn/trunk/demos/mapsapi/beefy.html