You'll be able to install the gui client for CP 2000 on a win2k machine, but you might have to install it manually or even download it from the website (http://support.checkoint.com) as the management server and enforcement point components won't install on win2k from some of the CD's (which makes it throw an error when it checks the OS) while CP says that the enforcement points and management servers can be installed on win2k machines. In NT you'll need to be running NT server with at least SP4.

However if all you want to do is put the gui client on the windows box, you can do that on just about any version of windows that is 9x or later.

hope this helps