The Reconnect Partnership is program involving Dell and Goodwill. They work together to recycle computers. They use Goodwill as collection centers like they already are for many things. Just go to the site and enter your ZIP to find a center near you.
Alternatively, find ways to reuse, repurpose or rebuild what you have. An older desktop in our house has just been rebuilt using Ubuntu rather than buying a new machine. It's like a new computer now and it was very easy. Several other ideas are available for older desktops that we may implement. Building a network storage or server, a server dedicated to Skype or a media server/PVR are possibilities. There are plenty of instructions online and software to do these things are readily available. It could even be a fun family project.