Here's the fastest way.
Download this nifty little Gadget for Windows and install and then set it up according to the instructions that come with it- http://defaultsound.tofran.com/
This works for Windows 7 straight off the bat. For Windows 10 there's one more step-
You will need this GadgetPack app that will allows Gadgets to run on Windows 10- http://8gadgetpack.net/
Install the the GadgetPack and then enable the Default Sound gadget through that.
Now you can switch between speakers and headphones straight off the Desktop with a single mouse-click!