Backspace takes you "Back". If you open C:\ and then click on Downloads, pressing Backspace will take you to C:\ and pressing Alt-Up will take you to C:\Users\YourUserName. Of course, if you are currently in C:\ and open the Windows folder, pressing either Alt-Up or Backspace will open C:\ again.

Actually backspace is the equivalent of pushing the back button. Which is often similar but can give different results. Boy I miss OSX sometimes :)