Blocking websites

Hi

I have one windows 2003 server in which I have running Routing and Romote access Server ( RRAS).this is for our Routing and port forwarding.Also I configured DHCP in the same.

The Server has two NICs one connected to DSL Router and other to LAN.through DHCP I assigned DG as the LAN IP of the server for Clients.No I need to block the site that is harmful to the organization also block the Internet as a whole for a perion say 8:00 AM to 7:00 PM allowed and others timing shouldn’t.

So I checked RRAS for this but no any result.is it possible if yes kindly give instructions