Git: Command not found Windows
You need to have C:\Program Files\Git\cmd in your path. Make sure it is there, start a new cmd prompt, type git --version.
As LaGrandMere explained, it's because git is not in your path. Instead of downloading and installing git, an easier alternative might be to run the cygwin setup.exe, and select git there. It will go into a directory that's already in your path (/usr/bin) and you will be able to run it from a bash prompt.
The one you installed works with the explorer, but that doesn't require the command to be in the path.