Wndows 2003: Proxy Settings config file

Can anyone hel;p me with a good proxy settings config file

I want my users to be able to browse certain sites like facebook and others only during lunch time between 12 and 2 pm and fro 5:30pm to 9:00 am.

Can anyone help me. I don't have a proxy server. I want to implement it as part of our group policy.

We are using message labs but have found out that it slows down internet browsing so i have decided to disable it until it is properly sorted.