Hi Cushtie,
Windows 2000 Server usually comes with a small number of CAL's (Client Access License) included, usually 5 or 10. These are separate to the license for the server operating system itself. There are two types of CAL: "Per Seat" or "Per Server".
With "Per Seat" licensing you acquire a CAL for each workstation or other device that connects to any server.
Client computers are allowed access to any server within a Windows 2000 Server-based network, as long as each client machine is licensed with the appropriate Windows 2000 Server CAL. So with this type of licensing 100 "Per Seat" CAL's means those 100 client PC's can access any number of Windows 2000 servers. This is the most economical licensing choice when you have client PC's accessing multiple servers.
With "Per Server" licensing you have a set number of devices connected to a server at a given time and need a sufficient number of CALs for that server to cover all the concurrent users.
A Windows 2000 Server CAL is associated with a particular server. This alternative allows concurrent-use licensing: If you decide to use the server in Per Server mode, you must have enough CALs dedicated to that server to accommodate the maximum number of clients that will connect to that server at any one point in time. So with this type of licensing if you had 100 client computers accessing one server, you need 100 CAL's for each server. Where this might be advantageous is in a single server situation or where you have many client PC's but not all access the server at any given time. This kind of CAL can be tricky because, in effect, you need to estimate the maximum number of users that are going to be connected at any given time. This is not necessarily the same as the number of PC's in the office; it's the maximum number of active connections to the server.
To simplify, the alternatives are:
1)to license each client to access as many servers as they like: Per Seat
2)to license each server for the maximum number of users connected to that server at any time: Per Server
Windows 2000 Server asks what type of licensing you want during installation. A dialogue box in the setup program of Windows 2000 Server asks what type of CAL license ("Per Seat or "Per Server") you want, so you need to have thought about this before installation. I'm not sure if this can be changed afterwards. But there is a place in Windows 2000 Server where you can check the number of CAL's, you can access it from the start menu, it's a menu item called "Licensing". Search for "CAL" in the help feature within Windows 2000 Server to find out exactly how to get to it.
The full details of licensing for Microsoft Windows 2000 Server are here: [broken link removed]