If you have an old machine that no longer has an operating system, you don't have to throw that machine away. You can put it to good use with an operating system that doesn't require a fast processor or big disk drive. Fortunately there are plenty of Linux distributions that will work on your old hardware and run at fairly medium speeds even if your machine is really old. There are quite a few Linux distributions that don't demand too much resources so that your computer will run relatively fast.
First of all, you need to stay away from distributions that use Gnome and KDE. These tend to use too many resources for old hardware and will cause the machine to lag way too much for basic applications. Look for these distributions that don't use these graphical interfaces.
These low resource distributions are all based on Ubuntu meaning that you have access to all the debian packages that Canonical supports giving you access to tons of software.
Xubuntu is a quality distribution that doesn't use too much power. It's uses XFCE which will help with slower computers. This one does work well but might not be best for very, very old machines as XFCE is a bit more demanding than some other low power interfaces. This will be a bit easier because it's basic interface works a lot like Windows which most people will be used to.
Linux Mint 6 Fluxbox Community Edition is a very high quality low resource distribution. This uses Fluxbox which will work better with computers that can't run Xubuntu too well. This distribution has all your basic essentials. It's layout will take a little while to get used to. Instead of a start button, you have to right click on the desktop to open your applications.
MoonOS LXDE is another good one to try. This uses LXDE which hardly uses any ram to actively run. Typically you can download LXDE but MoonOS has it all set up for you already after installing so that nothing has to be set up.
By following these tips you may be able to put an old machine to good use.