Originally posted April 2013. Updated April 25, 2016:
Microsoft, your Internet Service Provider (ISP), nor any legitimate company will call to tell you that your computer is infected, and that they need to remotely access your computer (for a fee) to fix it. It is a scam by crooks who are trying to access your computer, scare you into believing that there are multiple problems with it, and demand money to pay for a problem that didn’t exist in the first place.
Another common scam happens when you initiate the call. Don’t call a number that pops up on your screen unsolicited. If you call a company whose number you found from a web search, make sure you visit the company’s official web site – don’t click on an third-party ad. There are hundreds of fraudulent companies advertising on Google, Bing and other search sites masquerading as “tech support” for legitimate companies, and attempt to convince callers that they need to pay hundreds of dollars to “fix” multiple problems with their computer.
Never allow someone you don’t know remotely access your computer. For more detailed information please read New twists in tech support phone scams, Learn how to spot scams that pop-up on your screen, and the Federal Trade Commission’s information about Tech Support Scams.