Sometimes, you might want to install Linux on a really old computer. If this is the case, you will most probably not succeed in installing the latest and greatest desktop oriented distributions with all the bells and whistles, desktop cubes and shadows. Your old 486 or P100 with limited RAM just is not enough for all the modern eyecandy and probably unnecessary services running in the background.
If you are willing to try some less known distribution with less functionality, you certainly will be able to install Linux in just about any old computer you have in your garage or basement. You just have to be ready to learn something new and not expect to get everything running without user intervention.
In Ubuntu discussion forum, the user darreljon has collected an extensive list of lightweight Linux distributions that can be used even with very old computers. Some of the distros are not very current, some of them are still active. Check out the list and you might find a distribution that satisfies your needs.