If your network is not an active directory network you can create a registry entry file and merge it into your systems' registry.
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 18 United States Code 1030 et al. 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!"
You can of course customize the law from this website http://www.usdoj.gov/criminal/cybercrime/cclaws.html
The registry hack above is for Windows 2000, Windows XP, and Windows Server 2003.
You can cut and paste the registry hack and paste it into a text file and saving it to something like logonbanner.reg this will enable you to merge it into your registry.