People often want to use old DOS (or even Windows 9x, using the 4BIZ edition) programs on their current , faster, computers. So, they grab the discs, install it, start it up ... and then watch it die with a Divide Error (caused by code which was, ironically, designed to allow the program to run properly on faster computers) or run far too fast to be at all usable. This is especially a problem with many old games.
Mo'Slo is a solution for this - all it does is slow the programs down, causing the errant timer loop which causes Divide Errors to run properly instead, and let the program run at a sensible speed - and as such, it is a lifesaver for many of us.
It was developed by Dr. David's Super Crisp Software (no, I have no idea why the stupid name, either), was originally released in 1990 in response to the speed of the 386 chip, and there now are two commercial versions available - Mo'Slo Deluxe and Mo'Slo 4BIZ - as well as the freeware version, although they call the it an 'evaluation' and warn it may not run on computers running at speeds above 700MHz.
It's compatible with about anything, runs fine under Windows, DOS or even PC emulators, and has a very low memory footprint. It works by simply forcing the computer to be idle for a certain amount of time every few milliseconds. It's adjustable, the freeware version from 1% of CPU speed to 100% in 1% increments, and the commercial versions with even finer detail. Another useful feature, only in the commercial version, is the ability to change the slowdown percentage while the program is running.
Grab it now from http://www.hpaa.com/moslo/, and then go and get some abandonware or something to use it with!