I have had this problem as well. I like using 2014 but everything opens with 2015 instead. What you need to do is right click on the file. Go to properties then click "Change" where the Opens with option is. Browse to find the 2015 location (Program Files\Sketchup) rename the start up file and add a 1 after it so it becomes "Sketchup1" Now locate the sketchup 8 folder (Program Files (x86)\Google\Google Sketchup click the Sketchup application. Hit open then OK and Apply.
From now on any skp file should open with Sketchup 8.
Hope this helps
dave