I'm just about to install the Gnome 2.21.2 version of PCLinuxOS on an old machine today so that'll be interesting.
tickmike - MD5 is a hash function. Hash functions are one-way (in theory*) mathematical functions where it is impossible (in theory*) to determine the input to the function from the outputted hash. In practice this means that you can't alter the data put into the function such that you get the same MD5 hash output from the function.
For things like this the MD5 hash is used to verify that what you've downloaded is EXACTLY what was originally put in the download location - ie it verifies that nobody has altered the distribution iso file at a later date or in transit to you, which is more for security reasons than for failed download reasons. You don't "do" anything with it unless it doesn't match, in which case you try downloading again and if the MD5 hash still doesn't match then you should contact whoever is responsible for the distro to find out what is going on
*in practice there are "hash collisions" which mess the theory up somewhat. One day I might resurrect/update the encryption articles I did for The Caretaker's site a few years ago. If I get very very bored indeed