Don't do anything, just donate. And now for the long answer: If you will build and maintain a lab, use GNU/Linux (it really is best for education on many levels, except for cases where the necessary software is unavailable, which is becoming less common). I prefer Fedora on newer machines and Scientific Linux on older ones, but if you're more experienced with Ubuntu it's a great choice too (I maintain an office and two small educational labs in the Philippines that only use Linux, there are drawbacks but the benefits negate them for us). If you will donate machines to someone else, unless you are also donating a 'blank check' amount of support, leave Windows on it. If they don't have the ability to manage Linux machines, you won't have caused them any difficulty, and if they do know how to manage Linux, they certainly know how to install it (lately that's the easiest part).