The installer lists Gitorious as one of its packages. Grab that.
Learn Bash, from any Linux tutorial
Open a Bash shell (a "dollar prompt", like $).
Run cmd /c start . to figure out where you actually are
cd to where you need to be (but outside your rails project.
Run git clone git://bla/bla/blah.git
That will give you the clone, and a system to instantly install all the tools we use around here, and it will teach you a little Bash, which in my exalted opinion is more user friendly than Windows.