Clam AV (http://www.clamav.net/
) is according to the website, "an open source (GPL) anti-virus toolkit for UNIX, designed especially for e-mail scanning on mail gateways." It has build-in support for mail formats, archives and popular document formats, but it is not a all-in solution like mcafee or norton. It does not check the sites you are browsing.
If you want to use it, there is a pbi for pc-bsd : http://pbidir.com/bt/pbi/247/klamav
) on the other hand searches for and removes trojans from your system. But : TrojanHunter runs on Windows 2000, XP, Vista and Windows 7 (32-bit and 64-bit)... So I don't know if this is the program you are talking about.
But for safe surfing, I would indeed look for Firefox add-ons. There are plenty that do a great job.