1)Use it to store some of your music collection, network it and access those tunes anywhere in the house.
2)Use it as a firewall if your router doesn't already have one.
3)Run old games on it. The Wing Commander Kilrathi Saga is now available on the torrent network and is pretty good fun. The Doom games will play on it, as will Quake 1, 2 and probably 3. You might be able to run some emulators on it, or use it for ROM storage and use your main computer to play the emulators on.
4)Keep your install CDs for Windows and Office imaged onto its hard drive and then use a virtual CD driver to use them, that way you can make sure you don't accidentally let your optical drive scratch up the original CDs.
5)Connect it to your TV and hifi if it's got TV-out and stream video through it. There are plenty of TV shows you can illegally download
6)Use it to do that distributed processing thing...I can't remember what it's called but they're trying to get people to use their PC's idle clocks to find cures for things.
7)Take the HDD out and put just Windows and Program Files onto it, thereby your current PC's hard drive can be just for the things you want: games and media files.
8)Maybe take up graphic design as a hobby and use your main computer to do your normal stuff and leave the old PC to do the rendering whilst you conceptualise your next masterpiece.
9)Backup your data! Make your current PC backup all of your documents and photos to the old PC's hard drive once a day. I used to do this by making a batch file in Edit using the xcopy command and then making a Windows scheduled event to run this batch file once a day.