I need a free software that detects and removes viruses, Trojans and worms?

I have software that blocks new threats, and can scan and tell me of viruses, worms and Trojans that I had before I download the software, but nothing erases the old. I have PC tools spyware doctor software. I have to download Google.com package when I've updated Firefox., all the recommendations, reliable, good free software?

You can find a complete list of security tools reliable here: http://www.free-software-catalog.com/catalog.php?CategoryID=1

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