The other option is to create rules in your router configuration. Depending on your router, you can edit the ACL to either deny all outbound internet traffic - port 80, or to allow for specific outbound Internet traffic – port 80. (The sites you need to allow). This needs to be the first rule in the List.
Your other option, if you have fewer users/clients, then you can also configure the Internet explorer's content advisor feature to only allow the specific sites. Once the configuration is done, the administrator can lock down this configuration with a password for restricting further unauthorized changes. But, again this is not a foolproof solution as the users familiar with registry editing can tamper with configuration thereby by-passing the security restriction.
I would suggest you go with the first solution, as it is more secure, reliable and scalable.
This was first published in April 2005