Anti-virus software scam

In an anti-virus software scam, the victim receives pop-up ads for rogue anti-virus software. Victims reportedly receive ads warning them of the existence of threatening viruses and/or illegal content allegedly found on the victim’s computer. When the victim clicks on the fake pop-up ad, malicious code is downloaded onto their computer. Victims are directed to purchase anti-virus software to repair their computer, but in some instances this resulted in viruses, Trojans, or keylogging software being downloaded onto their computer. Attempts to contact the anti-virus software companies generally are unsuccessful.

Source
Internet Crime Complaint Center, 2009 Internet Crime Report.