"Hello, this is Internet Security Services. We're calling because we have detected a virus on your computer. Tell us your credit card details and we'll remove it. "
Never trust any unknown caller who says they can see what's on your computer. They're lying, they're a hacker, or both!
The fraudsters say they have detected viruses on the user's computer, and offer to remove them for a fee, payable by credit card. If the victim goes along with the suggestion, the scammers will use a perfectly legitimate web-based remote control service, as used by genuine computer support services, to establish a remote control connection to the user's computer; this can only be done if the victim co-operates. A genuine support service will establish remote control in such a way that the user can see what's being done to the computer, but the scammers may try to prevent the victim seeing what is being done, in which case they can install permanent remote control software and do whatever they want with the computer in the future. If the victim can see what's being done on screen, the scammers will probably just delete a few files from the Windows directory, in order to give the impression they're actually doing something. Eventually, the scammers will ask for credit card details for payment. They might not actually ask for very much, so that the victim is happy to pay the price. But of course, once they've got your card details, they can take whatever they want, and they'll do just that.
The solution is simple - never co-operate with anyone who phones up and says they've detected a problem with your computer. If you think the person calling might be genuine, ask for their name, company name and phone number, and say you'll call back. If it actually is your Internet service provider calling, you'll be able to verify their details and get back to them.