Ad-aware by lavasoft is good and they have a free downloadable version. Keep your definitions up-to-date and it will clean up spyware/data miner crap.

Ad-Aware will quarantine viruses, too, but you want something that will stop viruses before you stupidly execute them. In that case, you can pay for McAffee or Norton, or go with one of the better free versions like Cyber-Defender.
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She was standin' alone over by the juke box, like she'd something to sell.
I said "baby, what's the goin' price?" She told me to go to hell.

Bon Scott - Shot Down in Flames