Comment A desktop linux distro is a perfect choice. (Score 2) 360
I would go with Ubuntu, linuxmint or some such desktop distro today. Not so much because it's a better OS or will be supported longer than XP but because when you're donating computers you have to expect that they will end up in the hands of people who can't necessarily afford to buy software to do a lot of things and by default XP comes with 0 extras and won't necessarily know how/where to fine open source options for windows. In this light your typical linux distribution comes with software that will do a little bit of everything from office/word processing/whatever to editing graphics and even a few fun games for kids with plenty more for free in the repositories. I did this for several years in central Nebraska. In all I gathered up several hundred PC's from local businesses that were going to discard them and refurbished them and installed a user friendly linux distribution (at the time I was using Lindows/Linspire) and never had a complaint or even a call back with anything other than a thank you. Most distros like ubuntu, mint, fedora etc today are just as easy to use as windows out of the box even for a new user poking around trying to figure things out.