Locking down a network. Stop users from just plugging in.

Is there a way to actually authenticate each pc that connects to a network.  What i mean is we have a pretty large network about 200 local users.  We also have a bunch of ports in the building that are just sitting live for anyone to plug into.  I understand i could disable these ports to prevent that, but what about if an unknown user just walked in and unplugged another pc and plugged his laptop in?

We have a windows based network 2003 and 2000 with a windows dhcp server.  

Is there software? hardware? Let me know if there is anything i can explain about my network.

Almost all users are connected via cisco switches... but we do have 2 older dlink 48 port ones.

This all came about as i am creating a wireless network private and public to secure that... but then i thought about the users just plugging in.

any help would be great!!

Chad