i have other suggestions how you can use it:
1. the very first thing that comes to mind is to use your old computer as a proxy server. Not only this would save you bandwidth, but because caching it will also speed up your browsing experience.no matter how old that PC is, reading and sending data from its hard drive will always be faster than your Internet connection(unless you are in Norway or Sweden.)
http://www.proxyplus.net
http://www.handcraftedsoftware.org/
http://www.squid-cache.org/
2. Another good use of the old computer is to use the old box as a file server. this is specially good for home or small house where multiple people are using the same files. all you need to do is that make sure your old PC has windows installed on it and file and printer sharing enabled and all your shared documents are stored in a shared folder and remember to dollar share this folder as documents$ instead of just documents to prevent your old PC from being infected by viruses over the network. also install an anti virus which has real time scanner to protect all the users.
if you have Linux on that old PC then have a look at http://www.freenas.org/
3. one very popular use for old computer is to use them as a rather large mp3 player. for this your old computer will need to have a decent sound card with speakers connected. you can use its hard drive to store all your songs. you can use remote desktop connect if you don't wanna use it's monitor.
you can even stream you audio over the LAN with logitech's slim server(http://slimdevices.com/pi_feature.html) this open source software can be used by software players such as winamp, itunes etc. to listen to streaming music. all you need to do is to install it on your old computer, navigate in a browser to http://127.0.0.1:9000/ and start configuring . just make sure that you follow the online installation guide and look at the FAQ's to understand everything .after you install slim server make sure to go to server settings > security page and assign a user name and password to access the stream. you will also need to know your old pc's ip on the net to connect to it remotely.
you can use it to stream video also with vlc media player.
4. Old computers consume very less power than newer ones so you can keep it on 24x7 without worrying much about electricity use and then use it to download torrents or other downloads which take time and hence consume power. just unplug the monitor and then it would use even lesser power.
you can mix it all up and try what you want to do with that old pc.