You don’t want to just do a search for security software. It’s quite possible that you’ll find a tool that is useless, or even is malware itself.
Thus this excellent list of security software at Gizmo’s Freeware. Several of these tools are ones I use and recommend. There are, however, a number I haven’t used, so I can’t speak personally regarding every one. Still, it’s an excellent place to start.
(HT: Sun and Shield)