set url set for creating withelist when connecting to isp through web proxy

hi

we have isa server 2006 and i wana to limit users access to just some urls like msn or google by the way we connect to our isp with proxy server that our isa send all web trafic to that proxy server .when i make white list users cant browse withe list cause proxy address didnt mention in the withe list and when i add proxy server ip address then white list made none sence and users can do brose all internet site.

please help me to limit access of users to just few website through isp proxy server

THANKS A MILION

DARIUSHk