Its common these days to have old desktop pc’s (or even laptops) which are broken down or malfunctioning sitting in the corner… degrading… wait no, they can’t do that! But don’t despair! You can do something useful with them!
Big lists have been made up of possible things to do with old computers and these can be seen here: http://www.shadowlive.net/?p=15
Mind you, even that list is not exhaustive. You could even make it into a gaming server for garry’s mod that you can your friend can play on or just a small web developer server. I would eventually like to expand PortWiki to include guides on creating such servers.