When you're in Windows, see if you're logged in first with the following console commands (use Cygwin, not the Windows terminal) :
git config --global user.name
git config --global user.email
You need to be logged into a Git account with access rights to that repository. Use the same commands you'd use for UNIX but in Cygwin.
If you're using Github, there's a nice little GUI for it, as well as a UNIX-like terminal. The following download should install both.
http://windows.github.com/
Once you can "log in", click the grey checkbox next to this answer so that other people know it works.