Is it only single click opening apps/files/folders, or does it act as a double click when you click other things as well?
Check your keyboard and make sure that the command button isn't being held down somehow. Likewise, your keyboard could be going bad and is shorting out the command key. I'm not sure how to diagnose this, other than swap command for shift in OS X System Preferences and see if it still happens.
A shift-click will highlight multiple items in Finder. Swap command for shift by going to System Preferences > Keyboard > Modifier keys and for command, select shift. For shift, select command. Then open Finder and start clicking files. If it opens the file instead of just highlighting it, you still have the problem. However, if it highlights one item, click another and see if it highlights that one too. If it does, its a keyboard hardware problem.