Bem, podes usar um programa externo tipo este
http://leithauserresearch.com/Internet_Password_Lock.html
Ou simplesmente criares uma conta guest, restringires o acesso via DNS e voilá...essa conta nunca redirecciona todo o tráfego para o localhost. Se quiseres dar acesso a algum site basta editares o ficheiro hosts.
Citando:
Login as the administrator and set the DNS server address to 127.0.0.1
Now let the user log in as the guest account or the restricted
accounts (this prevents the user from having access to the TCP IP
control panel of the network interface I believe). When the user loads
up IE or Firefox they will not be able to resolve any domain names as
their DNS server ip settings will be incorrect and thusly cause
resolution to fail.
If you want them to have access to certain website you can edit the
local HOSTS file (located C:\WINDOWS\system32\drivers\etc\hosts) and
add entries for the website you require the user to have access to.
In my example I have set the following as an entry in the hosts file
127.0.0.1 xxxtoolbar.com
When I try to access xxxtoolbar.com (a spyware IE toolbar) it resolves
to my local PC and fails to connect. If you wanted to give the user
access to that website you would change 127.0.0.1 to the external IP
address of the domain name and the user would be able to access the
server fine.
If this is the information you required let me know and I'll post it as the answer.