Internet access provider EarthLink and security software maker Webroot scanned nearly 421,000 computers for their April Spy Audit report. Trojan horses and system monitors accounted for 133,715 pieces of the spyware found on those computers--representing almost one in three machines.
System monitors track users' computer activity, capturing virtually everything they do online. Trojan horses appear to be software programs a user has requested but actually aid hackers in stealing computer data. That information is then used to gain unrestricted access to users' computers while they are online.
"Consumers should be aware of the applications and files residing and running on their machines," Matt Cobb, Earthlink's core applications vice president, said in a statement. "While certain types of spyware are malicious, other programs can be used to improve their Internet experience."
Security experts note that the damage from Trojan horses or system monitors can sometimes be more severe than adware and adware cookies typically found on PCs.
When adding all four types of spyware found on the scanned computers during April, the Spy Audit Report found 11.3 million instances of spyware on the computers. That averaged 26.9 pieces of spyware per machine.
, 237,200 PCs were scanned, with 7.1 million pieces of spyware found on the computers. That averaged 30 pieces of spyware per PC, according to the report.