In many post-apocalyptic stories, people are using whatever vehicles they can find for as long as they can last. The point of the question is similar -- how can

In many post-apocalyptic stories, people are using whatever vehicles they can find for as long as they can last. The point of the question is similar -- how can you use whatever computers you can power for as long as they last? I'm not talking about rebuilding fab plants and stuff first; I'm talking about using the millions/billions of devices that are still around and working, and making them do something useful.