how to add disclaimer in MSExchange 2000

Disclaimer on E-Mails
I have 400+ e-mail users and MS Exchange 2000 server. The users use either Outlook 2000 or Outlook 2002. I need to set a standard disclaimer which will automatically appear on the bottom of each e-mail. Can this be done through exchange 2000 or has it to be set in a signature for each user. If it has to be set as a signature how can I stop users changing it. If anyone has any ideas, especially on the exchange 2000 option (if it exists), I would be very grateful.