Filtering Software

Hi folks, is there any working filtering-software for Squid, which stops the LAN from fetching URLs with pornographic and/or securityrisk-relevant sites (maybe Warez and especially virus-containing sites) ? If not, maybe there is any good filtering-software for the clients (running W2k). The problem is, that I have no budget for this :-( God damn the ignorant bosses... So I need a free software. Is there any ? thx. Crest