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Firms with anti-adware policies find it hard to stop cash filtering to adware companies, the US-based Center for Democracy and Technology (CDT) said.

Millions of domestic computer users around the world regularly find their machines infiltrated by programs that reduce performance and display streams of unwanted adverts.

Although some users choose to download the adware programs, often under false pretences, many other programs self-install themselves on a PC and are hard to detect and uninstall.

In a carefully worded report, called Following The Money, the CDT blamed major advertisers for being at the apex of a complex network that fuels the spread of adware and is not accountable for its actions.

"Knowingly or not, these companies are fuelling the spread of unwanted programs that clog people's computers, threaten privacy and tarnish the Internet experience for millions," said Ari Schwartz, deputy director of CDT.

The report highlighted recent comments by a US Federal Trade Commissioner, Jonathan Leibowitz, who described major firms' complicity in the spread of adware as a "dirty little secret".

The report's authors pointed out the confusing relationship between advertisers, advertising agencies and online distribution networks that act as the middle-men between advertisers and adware makers.

"Without advertising dollars, there would be no nuisance of harmful adware. CDT is committed to working with advertisers to stem the tide of this nefarious form of software," said the report.

Through a series of cash-based relationships, often involving "blind" deals that stop firms knowing quite how their advertising dollars are actually spent, adware firms are kept afloat thanks to some of the large sums of money spent on legitimate advertising, the CDT concluded.

Ben Edelman, a Harvard consultant working with the CDT, unearthed 20 companies whose adverts were distributed online by a well-known adware company, 180solutions.

The CDT then contacted 18 of those companies to ask whether they had a policy that would prevent the placement of their advertisements by companies that spread adware on the internet.

Among those named in the CDT report, at least one company, mobile phone retailer LetsTalk, said it would end its relationship with 180solutions.

For its part Netflix said that adware tactics were difficult to police but a spokesman told the Washington Post the company was "very vigilant" about the issue.

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