We need a better sound system in the gym...dontchathink?

The old sound system was pretty good...it had a subwoofer and surround sound, but the power supply crapped out, so all we have now is a couple little PC speakers.

Although I am a techno-geek, I am not much of an audiophile, so I don't know much about current stereo systems.  I have several devices that will play digital media files and I have some decent speakers, but I need a receiver to go between the input and output.
 
Do you have an old stereo receiver that will work?  Hopefully something cheap or free?

Do you have other ideas how we can improve the sound system?