blocking the internet (please help)

Hi i have a 600 users network and I need to prevent some users from surfing the internet and i need to set it on the user pc. i work in a hitech factory so the users here smart.
the users have windows 98 through xp.
i dont want to use my firewall or the gateway to do that cause if i`ll do that the explorer will ask for username and password and the users here as i said before smart and they will get a username and password from someone. so i want to block the internet localy but i need also to give access to some websites so the user can surf only to those sites and no more.