The worst-case scenario used to be that online ads are pesky, memory-draining distractions. <A HREF="http://www.wired.com/techbiz/media/news/2007/11/doubleclick">But a new batch of banner ads is much more sinister:</A> They hijack personal computers and bully users until they agree to buy antivirus software. And the ads do their dirty work even if you don't click on them.

The malware-spiked ads have been spotted on various legitimate websites, ranging from the British magazine The Economist to baseball's MLB.com to the Canada.com news portal. Hackers are using deceptive practices and tricky Flash programming to get their ads onto legitimate sites by way of DoubleClick's DART program. Web publishers use the DoubleClick-hosted platform to manage advertising inventory.