A. Regclean is a Microsoft utility which is used to tidy up the registry of a computer. When you install and remove programs sometimes registry keys/associations get left behind which just waste space and Regclean attempts to identify these and remove from the system.
Regclean.exe can be downloaded from http://download.microsoft.com/download/win98SE/Utility/4.1a/W9XNT4/EN-US/regclean.exe
Version 4.1a of Regclean build 7364.1 is compatible with Windows 2000. There are some issues running it on NT 4 Service pack 4 and below but if using Windows 2000 none of these apply and all should work as documented.