Disclaimer on E-Mails

I have 40+ e-mail users and MS Exchange 5.5 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 5.5 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 5.5 option (if it exists), I would be very grateful.