By the end of the 90s, the number of spyware packages appearing was starting to get seriously out of hand, so much so that in 2000 Gibson Research released a program called OptOut, designed to uninstall some of the major spyware packages. Soon after, a small European company called Lavasoft started offering software that removed many more of the most common spyware packages bundled with free software. Many users were surprised to find out exactly how much malicious software had been installed on their system with the programs that they had downloaded. 

Since Lavasoft introduced Ad-Aware, many other players have gotten into the spyware removal business, and it's now a legitimate software category.
Anti-spyware is very much like a virus remover - it finds and (usually) removes any spyware installed on a system. Some of the better applications are as polished as the best virus checkers, others are still shaky.

It's only fairly recently that the big anti-virus companies have started to get involved in the anti-spyware game. Because most spyware packages are legally, if deceitfully, installed, players like Symantec and McAfee feared legal retribution from spyware companies. Recently, however, as anti-spyware has become such an important utility in the Internet age, the big players have begun to offer anti-spyware solutions as well.

How anti-spyware works

The techniques that anti-spyware software uses to find and remove spyware are many and varied, but they are generally similar to the systems used to detect and remove viruses.

Some methods of detecting spyware fingerprints include:

• Hard disk scans. The anti-spyware software checks all the files on the PC's hard disk against a database of known spyware packages. The best solutions use a mathematical process called a checksum or hash algorithm to ensure that the files on the hard disk are exactly the same as those noted in the database. The worst anti-spyware uses file and directory names as a detection method, which is a very good way to get false positives and miss spyware that changes names.
• Registry scans. The software looks for modifications made by spyware to Windows configuration files.
• Memory scans. The anti-spyware checks currently running processes for any that match known spyware.
• URL monitoring. The software keeps track of visited Web sites and monitors cookies and executed ActiveX controls, and compares the sites and controls to its internal database of known spyware networks.

If there's a match, the package will be noted and the user given the option to remove the offending software when the scan is completed. If the user chooses to do so, the files, directories and Windows Registry keys will be removed.

Anti-spyware comparison databases need to be updated regularly as new spyware software comes online - much like anti-virus packages have to periodically download new virus definitions. For this reason, paid anti-spyware packages often work on a subscription basis. A subscription to the service gives you the right to update the spyware database when you need to.