We've recommended The Cleaner from Moosoft for a long time. It has worked very well, but we've been less than impressed it's results lately, and are currently evaluating alternatives. I'm certain we'll soon be recommending another solution.
IMO, you should have an antivirus program installed, and updated often (automatically preferred), keep Windows up-to-date (again automatic updates recommended), and have either a software or hardware firewall installed. If you've done this you chances of getting infected with a trojan are very small, and you shouldn't need an additional anti-trojan program.