Blocking a site

I have a small network, 10 workstations, one win2000 server, one gateway/router/firewall that does my DHCP. I do not know this router well, and do not want to start playing with the config. Is there a way to block only one or 2 web sites, using win2000 server. We have only only small case of mis use and blocking 1 or 2 sites would solve the problem, so I would like to avoid installing a blocking/monitoring hardware or software solution, any help much appreciated. The blocking must be for all workstations)