(free version) AVG...along with Spybot Search and Destroy and Spyware Blaster, along with avoiding
Internet Explorer, has kept my machines bug free for years now.
Occasionally, just for additional measure, I'll go to Trend Micro and launch a (free) HouseCall to check it out from another angle.
My wife's 'puter caught a trojan about a year ago, despite all the same software installed (she's not good about keeping it up to date, however) and the trojan looped itself inside the AVG program. Insidious thing
HouseCall entirely removed the trojan while AVG was still tripping all over itself infected with it.