We had the issue on a recent install of Windows 8.1. It wasn’t my machine so don’t know all the details but I used the following link which fixed the issue right-a-way.

http://www.askvg.com/fix-unknown-characters-or-vertical-rectangles-are-showing-in-place-of-metro-icons-in-windows-8-start-screen-and-login-screen/

It came down to a corrupt font. This may have happened when Adobe Creative Suite 6 got install, but I don’t know that for a fact.

I downloaded the zip file from the link above. Extracted it’s contents and moved the font from the Windows 8.1 folder and put it into the Windows/Fonts folder. It overwrote the old one. When I restarted the machine the issue was completely resolved.

I hope this helps anyone else out there….