GitHub for Windows is a neat tool but suffers from a severe lack of documentation.
I find a project I like, click "Clone in Desktop" on the RHS and it takes me to a screen saying "Download GitHub for Windows" even though I have the damn thing already installed.
So ^^%$$% frustrating.
Mr. Google to the rescue and after a number of false starts (Google sucks more and more at delivering useful search results) I discovered that you need to log-in first on the GitHub site.
Then all is OK.
Going by the number of hits I found on this, 99% of this would be avoided if GitHub put a notice to that effect on the page e.g.
"You must first Login".