My computer came down again with the flu. No bird or pig flu but computer flu.

It still allows me to surf the internet. Does anyone know a reliable virus/worm/trojan fighter that would decontaminate my computer on line? Preferably free as I do have McAfee already. McAfee has unfortunately proved useless once again (this is the second time). The guys who service my computers on line haven't yet managed to decontaminate my computer.