In a stand alone enviroment, Group Policies apply to all user's of the machine. Additionally, if you're using broadband internet, via an Ethernet connection, its almost impossible to block all Internet Access without 3rd party software.

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Doug Knox, MS-MVP Windows Media Center\Windows Powered Smart Display\Security
Win 95/98/Me/XP Tweaks and Fixes
http://www.dougknox.com
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Per user Group Policy Restrictions for XP Home and XP Pro
http://www.dougknox.com/xp/utils/xp_securityconsole.htm
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"[0v0]" <no.address@nd.no.spam.com> wrote in message news:MPG.1dd458579f1459ca989890@news.individual.ne t...
>
> I want to be able to stop one user accessing the Internet, not all
> users, on a particular PC.
>
> Is there any way to administer this using GPedit.msc (or otherwise) that
> doesn't affect everyone?