Spyware is software that runs on your computer without your knowledge, and it covers a multitude of applications, from the harmless to the extremely dangerous.

Some is installed by shareware and freeware developers, allowing them to gather information about how their software is being used in exchange for a free product. Other types, often referred to as adware, are used by manufacturers to feed back purchasing information without the consent of the user.

But the most dangerous forms of spyware are those that can provide unauthorised access to your PC, allowing other people to view and alter private files. Referred to as malware, these applications include keylogger software, rootkit applications and Trojans.

The growing threat of spyware

Spyware in all of its guises is a growing problem. A survey carried out by Webroot found that 87 percent of home PCs in the US were infected with spyware, while the average UK PC runs 31 pieces of spyware.

Despite the widespread use of antivirus software to protect against bugs and firewalls to stop hackers, spyware is still able to find its way on to most home PCs. This could be because no antispyware software is being used, or because the free tool that is being used doesn't offer the most up-to-date definitions. In fact, some free antispyware software actually installs spyware, preying on user concerns over safety.

There are three main threats in this area at the moment: rootkits, which provide shell programs to hide backdoor access points to your PC; keyloggers, which record keystrokes, potentially revealing sensitive information and passwords; and Trojans, which can steal information and provide a means for spyware writers to install more malicious applications on to your computer.

Trojans have been around for a while, but they are becoming more sophisticated and difficult to track. They're targeting common websites in order to achieve the highest infection rates. Another new development is spyware attempting to prevent your antivirus software from updating definitions via the web. And even the more innocent adware is becoming trickier to find and erase, as its developers adopt more underhand practices to prevent detection.

Protecting yourself

Spyware has a detrimental affect on your computer, however it manifests itself – whether it takes the form of adware programs slowing down performance or malicious software hijacking websites, leaking personal information and providing unauthorised access to your PC. It's not enough to rely on antivirus software to keep your computer safe. A robust antispyware tool is a must.

To test the options available, we've pitted two of the latest contenders against one another: Webroot's Spy Sweeper 5.0 (reviewed here) and Grisoft's ewido anti-spyware 4.0.

Spy Sweeper has an intuitive user interface

Ewido took more than an hour to sweep our test PC, but it found 55 pieces of spyware