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Any company that claims your computer is infected, has been sending errors, or claims to have any direct affiliation with Microsoft is a scam if they are calling you or somehow getting in touch with you to repair your computer, fix errors, or remove viruses. Chances are good that your computer is running just fine, and they will charge hundreds of dollars for services you do not need.
Please avoid dealing with these companies and inform them that you know they are a scam and that if they continue to call, you'll report them to the Internet Crime Bureau, which you can file a complaint with here: http://www.ic3.gov/default.aspx
If you already paid, then I would file a dispute with your credit card company.