[NTLUG:Discuss] How do you secure a LAN?

[Bob Byron]

How do you secure a LAN?  Not every point of access is under my 
complete control.

I want to make sure that no one connects any PCs that I don't know
about to the company LAN.  What is the best way to secure it?  Ideally,
I would like to have the LAN setup to do DHCP, however, with a user 
name and password required to register with DHCP.  But, since that
is not possible (that I know of), I am open for suggestions.

Thank You,
Bob Byron