It's the "take off and nuke the site from orbit" option, but have you considered reinstalling from the partition/install disks?
I use Avast! which is free for home users, and haven't experienced any problems. Try installing that, letting it run its own check right after installation, see what it has.
Also, I'm not sure if I've misread your assessment, but cleaning your machine once a year is not going to provide protection. You gotta wrap that rascal before you take him out to play. (Kosma, this goes for you too!

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