An average of one in three PCs scanned in March and April was carrying a system monitor or Trojan horse hidden on its hard drive.

The research, carried out by privacy firm Webroot Software and ISP EarthLink, warned that these sorts of spyware can forward information about a user's online activities to another individual or company without the user's knowledge or permission.

Scans of 650,000 consumer PCs performed between 1 March and 30 April found more than 18 million instances of spyware.

Trojan horses are applications that appear to be genuine software programs, but instead hide secret and malicious code that can allow hackers to gain unrestricted access to computers while users are online.

"Consumers should be aware of the purpose of all the applications and files residing and running on their machines," said Matt Cobb, vice president of core applications at EarthLink, in a statement.

"While certain types of spyware are malicious, other programs can be used to improve users' internet experiences.

"After the discovery of harmful spyware, users need to take action to immobilise or remove the programs that they don't want on their machines."

Nick Lewis, managing director of Webroot UK, added: "Based on the overwhelming number of spyware traces identified in just four months, we continue to urge consumers to run an audit as soon as possible to determine if they have spyware on their PCs and then take action to manage it."