The way I understand it.
When you get Windows 2000, 120 days evaluation CD, and you set a Network it is usable only for a server and 10 computers.
Let assume that you are happy with the trial and you buy Windows 2000 with licensee to install 50 computers on the netwrok.
Under normal situation, you have to erase the evaluation setting, and reinstall every thing from scratch.
This site sells a piece of software that allows you to install a new legitimate Win2000 on top of the evaluation without losing your previous setting.
If you use a Stopwatch you can clock how much time it saves.