GPO: Restrict WEB access to users

Hello,

I'm trying to restrict the web acess to some users using GPO but i'm encountering a problem. I have windows Server 2003 and the clients are XP.

I enable the content rating on GPO with the approved sites but the following situation occurs:

User 1: Full acess > logs on to computer 1 > Have full access to the web
User 2:Restricted > logs on to computer 1 > Restrictions applies
User 1 > logs again on computer 1 > Restrictions applies

I think once a user with restricted access logs on to a computer the restrictions stay "bonded" to that computer. Is there any workaround this situation, using AD and windows services ?

Thanks