I use AVG Free 8.5, Lavasoft Ad-Aware, and Spybot Search & Destroy.
They recently saved me from a Trojan attack, the one with the "you must update your shockwave player" screen that stays above all windows.
I restarted the computer, didn't open up the internet browser (that's what triggers it) and ran Spybot, it found it almost immediately and removed it no problem. Then I restarted again, ran the browser, it was gone, and then ran AVG, and it did the mop-up.
Only bad thing about AVG Free is that it purposely doesn't let you scan for rootkits. But that's what my other software is for.