See
www.dougknox.com, Win XP Utilities, Windows XP Security Console. The licensed version will allow you to put this type of restriction, and others in place on a per-user basis. If your XP Pro machines are part of a domain, you can do this through Active Directory and Group Policies.
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Doug Knox, MS-MVP Windows Media Center\Windows Powered Smart Display
Win 95/98/Me/XP Tweaks and Fixes
http://www.dougknox.com