I just updated my computer to Windows 8.1 and while I was working on a spreadsheet in Google Drive, I suddenly pressed a combination of keys that made my computer start reading out stuff on the screen. Here is how I got it to stop.
Press [Windows] and [Enter].
This will also make it start again. I got myself into this mess by trying to make a line break in a cell in my spreadsheet (which is done with [Alt] and [Enter]) and mistyping.
More info is available on the Windows site.