Group Policy upon Computer Start-Up

Create a Group Policy for your logon banner or a registry hack.

Here's the reg hack I've used.

This will work with Windows 2000 Professional & XP.
Windows Registry Editor Version 5.00

[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon]"LegalNoticeCaption"=" !!!WARNING!!! !!!Warning!!! !!!WARNING!!!""LegalNoticeText"="This is a Company X computer system, which may be accessed and used only for official Company X business by authorized personnel. Unauthorized access or use of this computer system may subject violators to criminal, civil, and or administrative action under British & EU Law. Use of this system constitutes consent to monitoring, retrieval, and disclosure by authorized personnel. USERS HAVE NO REASONABLE EXPECTATION OF PRIVACY IN THE USE OF THIS SYSTEM.""Welcome"=":WARNING! Company X System!"

cut n paste the above into a text file and then save as a dot reg file - .reg.