Locking down computers

We have 4 computers that are running Win2K that our customers can use when they come to the office. We would like to lock them down so that they can only access our Intranet and our Internet web page along with not having access to our servers. I know I can use AD to set their limits to our servers but I am not sure how to lock them out of the Internet. any help would be greatly appreciated.
Thanks,