Last year I revived an old, underpowered Windows box by wiping the hard drive and just installing Ubuntu straight-up. Aside from the dramatic speed-up in performance, I can state from experience that Ubuntu is definitely wife-friendly - it was a smooth and painless transition for my not-nearly-as-geeky-as-me life partner. (It helped that we had already been using mostly free software even on Windows.)

As for git on Windows, I've found Msysgit to be a lot faster and more stable than git on Cygwin. You can also configure it during installation so that git commands work on the CMD command line, which is marginally less bad than Msysgit Bash.