Spyware refers to malicious software that takes partial control of a computer's operation through unauthorized access without the computers owners knowledge or consent, and has become one of the biggest security threat to computer users today.

Spyware exploits infected computers for commercial gain, usually by including unsolicited pop-up advertisements; by monitoring the users online activities; by routing http requests to advertising sites; or by stealing personal information such as financial information. In fact, it is estimated that identity-thieves have stolen over US$ 24 billion worth of account information in the United States alone.

Spyware most often infects a computer by piggybacking on a desirable software and the user unknowingly downloading and installing it. Spyware also often lurks in rogue anti-spyware programs that pretend to be security software. Spyware also comes bundled with shareware or other downloadable software and music CDs. Yet another method of distributing spyware involves tricking computer users by manipulating security features designed to prevent unwanted installations. For example, surfers using the Internet Explorer web browser may unwittingly click on a command prompt that appears to be a Windows dialog box, when in actual fact it initiates the spyware download.

With the spyware threat worsening, several anti-spyware programs have emerged that are designed to counteract or remove spyware. Programs such as Ad-Aware SE and Spybot - Search & Destroy are some of the more popular and effective tools to remove and intercept spyware programs. Microsoft has also released Windows AntiSpyware to combat the spyware problem. Other popular anti-spyware programs include Spy Sweeper, Spyware Doctor, XoftSpy, and CounterSpy.

Anti-virus firms Symantec, McAfee and Sophos, while reluctant initially to add anti-spyware features to their anti-virus products, have finally added anti-spyware features to the recent versions of their anti-virus products.

Computer users have also become savvier in detecting spyware. Many users now install a web browser other than Microsoft's Internet Explorer (IE), such as Opera or Mozilla Firefox which are not as vulnerable as IE is to spyware - though that is not to say that Opera and Firefox have not suffered from similar security vulnerabilities. No one single browser is 100% safe, as security all depends on the person using the browser.

Some internet service providers, especially colleges and universities, block spyware by using their network firewalls and web proxies to block access to Web sites known to install spyware.

Spyware sometimes hide in shareware programs offered for download, so downloading programs only from reputable sources provides some protection from this source of attack.