I recommend a Mac.

*runs*
Okay, seriously. I don't use them a whole lot, because I consider myself a smart browser. But the ones I do use is Ad-Aware and Spybot.
Trend Micro is a good free antivirus. For that matter, all of the big names (MacAfee, AVG, Norton) are all good, but watch out with them, as they can attempt to take over your mail settings, and actually prevent you from getting your mail. (Norton and AVG are the big offenders here). You can change the settings to stop this, though.