Think spyware isn't spreading? According to a new report from EarthLink and Webroot Software there's an average of almost 28 spyware programs running on most PCs. More seriously, trojan horse or system monitoring programs were found on more than 30 percent of all systems scanned, raising fears of possible identity theft.

The report presents the results of scans involving over one million internet-connected computers. Many of the 29 million spyware programs that were found were harmless "adware" programs that display advertising banners or track surfing behaviour. However, the companies involved in the report also found more than 300,000 instances of programs that are capable of stealing personal information or providing unauthorised access to computers.

Spyware is a generic term that describes a wide range of programs that track user behaviour on a computer, often for marketing purposes.

The programs are sometimes bundled with other software such as peer-to-peer file sharing programs and installed legally on users' systems. Once installed they run surreptitiously in the background and can be difficult to detect and remove.

The report covers the first three months of 2004 and compiles information from scans conducted by both EarthLink and Webroot. It is the first of what will be regular studies into the prevalence of spyware, the companies say.

A statement issued by both companies said that the results show the proliferation of spyware and should act as a warning to computer users, encouraging them to take steps to protect themselves.

The detection of over 184,000 trojan horse programs on the systems scanned and a similar number of system monitoring programs – such as key-logging software – underscores the potential for identity theft and system compromise for internet users, says a statement attributed to Matt Cobb, EarthLink’s vice president of core applications.