What is the best free anti-spyware and virus protection on the internet?

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i was just wondering in your opinion what is the best anti-spyware and virus protection(such as avast) on the internet that you have used...i have tried a few and they really slow my computer down i don't know what the problem is, i thought my computer but then others that used it said that it slowed theirs down too so i know it can't just be my computer. i was just wondering what everyone liked to use on their computer and does it slow your computer down at all and how good is it for you??