Some videos offered by advertising software maker Zango are likely being distributed without permission from the copyright holders, according to a spyware researcher.

Zango's advertising software shows pop-up ads related to the content users have been browsing. In exchange, users get access to videos and other online entertainment content on Zango's Web site that the company says it has licensed from the copyright holders.

However, some of the content there is not licensed, according to Ben Edelman, an assistant professor at Harvard who has criticized Zango repeatedly. He also wrote that his latest look at Zango's software also found that much of the content on the company's site is available for free at other sites on the Internet, and that some sexually-explicit videos lack proper warnings.

Edelman wrote that he is contacting those who own content offered by Zango, and some say they've not given permission to Zango.

"Zango should be criticized and shunned for reproducing others' copyrighted work without any apparent license to do so, showing sexually-explicit material unrequested and offering users a lousy value by bundling extra ads with content users could get elsewhere for free," Edelman wrote.

The company, formerly known as 180solutions, has run into repeated trouble over its software. It was ordered by the US Federal Trade Commission in November 2006 to forfeit US$3 million after it found the company's software was difficult to remove and deceived consumers as to its function.

Zango disputes the accusation, writing in a blog post that it has a team that secures permission from copyright holders and has agreements with 100 content providers.

Still, Zango acknowledged some of the same copyright-related troubles shared by others. Google, which owns YouTube, was sued in March 2007 by Viacom for US$1 billion over clips uploaded by users without permission.

"We occasionally do receive copyright-related inquiries about content available at Zango.com and via our syndication platform," the company wrote. "In each and every instance, we investigate those claims quickly and resolve them on a case-by-case basis."

One clip offered by Zango features the comedic character Borat. The clip bears a DivX symbol, a sign the clip was ripped from a DVD, Edelman wrote.

"No authorized reproduction would be provided with a DivX overlay, so the presence of the DivX marker confirms that this video was reproduced without permission from the creators of Borat," Edelman wrote.

Zango's programs have a huge install base. In April, Microsoft said one of its security tools installed on Windows PCs detected 7.1 million instances of HotBar, and 4.9 million instances of the ZangoSearchAssisant, another adware program.

Security software companies frequently label Zango's software as "potentially unwanted" program, but do not automatically remove it from user's PCs.