I've been getting a lot of calls about this recently. It's basically just a popup that appears on a persons computer screen while browsing the net. The person then (Most if not all the time) calls the number and then is persuaded to let the person on the other end of the phone remotely connect to their machine. I don't think they do anything bad to the machine or install any malicious software, they basically just open up things like the Event viewer and tell them everything they see is a virus on their computer.
They then try to sell the person a security package .. one of my customers said she gave them $600. Which some how she did get back and also changed her card number.
Every customer who has contacted me about this says, they have called the number, they did let them onto their machine. Some gave them money, other have gotten suspicious when they ask for money.
The person who gave them the $600 told me and I quote "I'm going to call Norton and have them explain why their antivirus let all these viruses on my system *face palm*