A number of patrons have fallen victim to a scam currently making the rounds. If someone phones
you, tells you they’ve detected all sorts of problems with your computer, and offer to help you get rid of them, HANG UP. They may sound knowledgeable and helpful, but they are after your money and the data on your computer. Do NOT give them access to your machine and do NOT give them any credit card info. These guys are very smooth and convincing, and will try to gain your trust. Don't let them.
This scam is widespread and ensnares many people who are justifiably concerned about their computer's security. The Federal Trade Commission has a good summary of the scam at this link.
If you have already given them your credit card number, consider calling your financial institution and canceling that card. If you gave the scammer remote control of your computer, consider downloading and installing one of the many free, legitimate programs that will help you get rid of any unsafe programs they may have left behind on your machine. I like malwarebytes.com, but there are many.