PCs could be carrying nearly 30 pieces of 'spyware' software secretly forwarding information about a user's online activities to others without their knowledge.
And industry experts suggest that these types of programs may infect up to 90 per cent of all internet-connected computers.
One million scans conducted by internet service provider EarthLink and privacy software company Webroot Software found more than 29.5 million instances of spyware on subscribers' PCs.
"This figure equates to an average of nearly 28 spyware items per computer and demonstrates the broad proliferation of spyware," said Matt Cobb, EarthLink's vice president of core applications, in a statement.
"While most spyware is adware-related and relatively benign, it's disturbing that over 300,000 of the more serious system monitors and Trojans were uncovered.
"This figure represents how real a threat identity theft or system corruption is for users."
Typically, spyware arrives bundled with freeware or shareware, or through email or instant messaging programs, and begins reporting the next time the user goes online.
Adware is software in which advertising banners are displayed while the program is running and which then sends data back to a third party without permission. Adware cookies allow a website or software to record a user's surfing habits without their knowledge or consent.
System monitors are designed to keep track of a consumer's computer activity and can record virtually everything a user does online.
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