This site states that:
"But consumers who have downloaded ad-supported software on the Internet will quickly notice pop-up ads from Travelocity, Priceline.com, Thrifty, United Airlines, American Express, Honda, DaimlerChrysler and Toyota. Retail companies such as Lancome and L'Oreal are experimenting with the trend, as are technology heavyweights like Microsoft, Oracle, Sprint and Verizon."

So it seems that downloaded software is probably the source of your problem

CHEERS
Nigel