is what you're after
Put that on a server somewhere and then programs like itunes will connect to it and be able to play the music from it across the network. Everything will work pretty much as before (it'll just show up as a remote library in itunes.)
If it were me I'd put together a low powered small form factor system (some of the new intel atom boards would be suitable - cheap and use very little power), whack a Linux environment on it without any graphical stuff and then setup firefly (and anything else you want) to stream your library. It shouldn't be difficult at all, but there are beta versions that run on windows if you're happier using that. I can't vouch for their stability though since I've never used them.