I also found just the first name is the actual username. This is important for certain third-party applications that borrow the login credentials from Windows.
The Windows 8.1 environment NEVER asks for your username. When it shows the 'username' it is in an email address form. This may not be the actual username (as the previous post shows, it may be just the first name portion), which makes figuring out what the heck these third-party applications are looking for... entertaining. I eventually guessed my own, since I had set up my Windows copy only a few weeks earlier, but I am running into this problem at several places. Thanks for the solution.
This reminds me of the obnoxious 'hide extensions for known file types' setting that invited all sorts of mislabeled viruses to get clicked on, which is annoying, useless, and potentially dangerous.