Search engines deliver links to dangerous websites that download spyware and adware to visitors' PCs, exploit security vulnerabilities and attempt to scam users and include them in spam lists, a study has found.

US users land on malicious websites about 285 million times per month by clicking on search results from the five major search engines, according to the study, conducted by McAfee's SiteAdvisor unit and released last week. Google, Yahoo, MSN, and AOL comprise the top search engines.

It also found that the adverts search engines run with their results are significantly more likely to lead users to malicious sites than unpaid-for results. On average, almost nine percent of these so-called sponsored results link to rogue sites, compared with an average of 3.1 percent of normal search results. Of the major search engines, MSN delivered the fewest malicious links, with 3.9 percent, while fared the worst, with 6.1 percent.

"Our core advice: it's a jungle out there. Users should be careful where they go and what they do when choosing sites based on search engine results," reads the study. "Users can't count on search engines to protect them."

Some popular search keywords such as 'digital music' and 'singers' can trigger a set of results in which 72 percent of the items link to malicious websites, according to the study, conducted between January and April.

Marissa Mayer, Google's vice-president of search products and user experience, acknowledged that this is "a real issue" and said Google has taken steps to address it. For example, Google offers an antiphishing plug-in for the Firefox browser, she said.

Meanwhile, a Yahoo spokeswoman said the company offers a spyware detection and removal tool in its browser toolbar and it also has systems and employees devoted to improving the "quality" of its ad listings. Users of its search engine can also report malicious sites to Yahoo, she said in an email.

Microsoft is also aware of the problem and attempting to curb it with its toolbar's pop-up ad blocker and antiphishing filter, said Justin Osmer, MSN senior product manager. "And we continue to work on all aspects of our search service to provide people with the best answer possible and protect them against malicious sites," he said via email.

That the vast majority of search results are safe is a major accomplishment, an spokeswoman wrote via email. "Like the other major engines, we are committed to raising the bar even higher as we continue to apply human processes and technology to identify and remove unwanted sites from our index," she wrote.

AOL didn't immediately respond to requests seeking comment.

McAfee suggests an unsurprising solution to the problem: users should download the free browser tool from SiteAdvisor, which tells users if websites are safe or unsafe to visit.