I had a customer that fell for a scam yesterday. Someone called him claiming they were from Microsoft and stated his computer was infected with a virus. They wanted to help him remove the virus and convinced him to install a program that gave the caller complete control of his computer. Bottom line: SCAM!!! If someone calls with such a claim HANG UP THE PHONE. Let me just say that Microsoft is not going to call you if your computer has a virus. They don't have a way to correlate your phone number to your computer (at least right now).
Click here to read what Microsoft suggests you do if you've fallen for something like this. If you are unsure you should call a computer expert.