I used to have a job installing and fixing computers and networks. Sadly, I saw a number of my customers fall for this scam. A pop-up window appears on your computer, advising you that you have a virus and that it can be fixed by downloading a company's virus removal program that costs $30 or $40. Many people believe all this is true and they fall for it. They purchase the product.
The problem is that it is all a scam. Your computer probably never had a virus. The pop-up is lying to you. Even worse, when you pay for the program, download it, and install it, these scams often install new viruses onto your hard drive! The new viruses will scan your hard drive, looking for passwords, bank account information, and more, then will send that information to the scammers' servers.
Organized criminal gangs are now are tricking security-conscious Internet-users into purchasing anti-virus software to access their bank details. If you fall for such a scam, you may see your bank account depleted within hours.
If you receive such a pop-up warning of a claimed virus, turn your computer off immediately! Then seek assistance.
You can read more in an article in eWeek at http://www.eweekeurope.co.uk/news/web-users-prone-to-fake-av-2-13384