Poor people don't suffer the network effect, sorta
Really old computers shouldn't go to nonprofits and charities, at least not for their own use, because organizations need computers that can run current software and network. They need to be able to use current websites.
It's better to give these to people who just need a computer, and are so lacking in a computer that running Windows 2000, or a small Linux like Crunchbang, is still a benefit. Generally, these people will eventually be fed up with the slow computer, but it'll be an educational experience that can spur them to save the $350 to buy a new low-end computer.