This is not an easy question to answer. Old computers are slow, potentially toxic, energy hogs. Throw them away? Keep using them? Recycle them? Its really hard to say - its nice that they are built out of components so that they can be recycled, but chances are they won't be. It will most likely be cheaper for most big companies to dispose of the machine and sell a new one.
But is it cheaper? Here's an article about how tech trash can be handled very badly: