So you've got an Xbox 360/PS3/Roku SoundBridge/etc. and you want to make use of the media extending features of it. The easiest way to do this is share your folders in Windows(XP, Media Center, or Vista), but this is really only a good option if you plan on having the computer on at the same time as your extender. So what if you want all that extra media to always be available?
Build a server. Trust me, it's not as hard as it sounds. You can put one together using your old parts, just as you would any other desktop computer, but instead of installing Windows or OSX, throw on a (relatively) user-friendly version of Linux like Ubuntu(For free! Legally, too!). Then it's just a matter of starting up a service like Samba to let your other computers talk to the server, and pointing a UPnP Server application in the direction of your files. Don't be daunted by this, you can take your time, and if you get it wrong, you're not going to be wiping out any files, and you're not going to be wasting any money to tinker around a bit. It also doesn't take that long if you like the default settings on some of these programs as well, and with a little guidance, you could could reasonably expect to finish this project in a day.
As far as UPnP servers go, I highly recommend MediaTomb, it works well with all the examples I mentioned above, it's fairly easy to use, and easy to navigate and administer from any computer on your home network.
That's just a taste for now, I'll provide links and greater details later this week.
