The best answer I can give is, "do you throw old books away when you're done reading them?" .. this always makes people think about it a little, and then I follow up with ".. well throwing away computers is like throwing away old books. Exept it took 10,000 barrels of oil to build the computer, another few thousand barrels of oil to get it to you, probably a few thousand while you used it, and it'll take another few thousand barrels to get rid of it while it rots".
Better to just keep it running and find a user. For every old, archaic, 'outdated' computer that people chuck into the land-fill for the sake of consumerist satisfaction, there are a thousand users who you don't know about who will put that thing to valid use. Find those users, don't throw away the computer.
Old computers never die - their users do!