I am thinking that when you say you want it to be a "Back" button is so you can get to a previous opened app or window.
Why not use the Task View on the task bar, this brings up a small version of all open apps or programs and even multiple desktops, then you simply go to the one you want, far easier than swiping through windows.
No, if you were thinking correctly then I would just swipe in from the left and use the task view (no need for the task bar, I am using a tablet as mentioned in my previous post). Problem is, all that can do is switch tasks. It cannot switch to the previous view in the same app. Try using the back button yourself (it is on the task bar in tablet mode, a big arrow pointing to the left, or you can use the backspace button on a keyboard), you'll find it is not really a button to access the task switcher but a button for navigating within an app. The task switcher button is a separate button on the task bar and it does not do the same thing at all, neither does it look anything like the back button.
I'm not trying to find a way to access the task switcher, otherwise I would be asking how to access the task switcher rather than cryptically talking about the back button. The task switcher is already easily accessed. I need to access the back button. Having to persuade you that I ought to access the back button is not really getting me anywhere. Please, just accept that I need access to the back button and if you don't understand what the back button does, just experiment with it. You will soon see why it is necessary.
Anyone have any idea as to how to do this? All I need to do is to redefine the tablet's windows key so that it acts as a back button. I believe it is possible with adjustments made to the registry but my research is not getting me to the point where I can see exactly what I need to do to make it happen. Any actual suggestions as to how to achieve this would be very welcome.