www.cygwin.com

Basically a virtual linux system that runs within Windows alongside everything else, including a bash shell, XWindows for graphic based linux programs, and even a Gnome port somewhere too if you look hard enough I've actually edited my PATH variabled for Windows, so I can run some of the tools form a DOS box rather than a bash prompt

I can't seem to find vi, but vim is present, that's kind of like an improved vi. Just watch the disk space, the installer and installed stuff is over 1 GB on my disk, but I have installed a heck of a lot more than just the shell and vim