Spyware is computer software that is installed surreptitiously on a personal computer to intercept or take partial control over the user's interaction with the computer, without the user's informed consent.

While the term spyware suggests software that secretly monitors the user's behavior, the functions of spyware extend well beyond simple monitoring. Spyware programs can collect various types of personal information, but can also interfere with user control of the computer in other ways, such as installing additional software, redirecting Web browser activity, accessing websites blindly that will cause more harmful viruses, or diverting advertising revenue to a third party. Spyware can even change computer settings, resulting in slow connection speeds, different home pages, and loss of Internet or other programs. In an attempt to increase the understanding of spyware, a more formal classification of its included software types is captured under the term privacy-invasive software.

In response to the emergence of spyware, a small industry has sprung up dealing in anti-spyware software. Running anti-spyware software has become a widely recognized element of computer security best practices for Microsoft Windows desktop computers. A number of jurisdictions have passed anti-spyware laws, which usually target any software that is surreptitiously installed to control a user's computer.

Signs of spyware: Are you being watched?

If your computer starts to behave strangely or displays any of the symptoms listed below, you may have spyware or other unwanted software installed on your computer.

• I see pop-up advertisements all the time. Some unwanted software will bombard you with pop-up ads that aren't related to a particular Web site you're visiting. These ads are often for adult or other Web sites you may find objectionable. If you see pop-up ads as soon as you turn on your computer or when you're not even browsing the Web, you may have spyware or other unwanted software on your computer.

• My settings have changed and I can't change them back to the way they were. Some unwanted software has the ability to change your home page or search page settings. This means that the page that opens first when you start your Internet browser or the page that appears when you select "search" may be pages that you do not recognize. Even if you know how to adjust these settings, you may find that they revert back every time you restart your computer.

• My Web browser contains additional components that I don't remember downloading. Spyware and other unwanted software can add additional toolbars to your Web browser that you don't want or need. Even if you know how to remove these toolbars, they may return each time you restart your computer.

• My computer seems sluggish. Spyware and other unwanted software are not necessarily designed to be efficient. The resources these programs use to track your activities and deliver advertisements can slow down your computer and errors in the software can make your computer crash.

If you notice a sudden increase in the number of times a certain program crashes, or if your computer is slower than normal at performing routine tasks, you may have spyware or other unwanted software on your machine.

How to help prevent spyware

Spyware and other unwanted software can invade your privacy, bombard you with pop-up windows, slow down your computer, and even make your computer crash. Here are several ways you can help protect your computer against spyware and other unwanted software.

Step 1: Use a firewall
While most spyware and other unwanted software come bundled with other programs or originate from unscrupulous Web sites, a small amount of spyware can actually be placed on your computer remotely by hackers. Installing a firewall or using the firewall that's built into Windows XP provides a helpful defense against these hackers.

To learn more about firewalls, read Why you should use a computer firewall and get answers to your Frequently asked questions about firewalls.

Step 2: Update your software
If you use Windows XP, one way to help prevent spyware and other unwanted software is to make sure all your software is updated. Visit Microsoft Update to confirm that you have Automatic Updates turned on and that you've downloaded all the latest critical and security updates.

Step 3: Adjust Internet Explorer security settings
You can adjust your Internet Explorer Web browser's security settings to determine how much—or how little—information you are willing to accept from a Web site. Microsoft recommends that you set the security settings for the Internet zone to Medium or higher.

To view your current Internet Explorer security settings:

1.
In Internet Explorer, click Tools and then click Internet Options.

2.
Select the Security tab.

For a step-by-step guide to adjusting your settings without blocking content from sites that you trust, see Working with Internet Explorer 6 Security Settings.

If you're running Windows XP Service Pack 2 (SP2) and you use Internet Explorer to browse the Web, your browser security settings for the Internet zone are already set to Medium by default. Internet Explorer in Windows XP SP2 also includes a number of features to help protect against spyware and many other kinds of deceptive or unwanted software.

Tip: Don't know which version of Windows your computer is running? Find out.

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Step 4: Download and install antispyware protection
Windows Defender protects your computer from spyware and other unwanted software. Windows Defender comes with Windows Vista and you can download it for no charge for Windows XP SP2. For more information, see Windows Vista: Windows Defender.

Additional security tools to help block, detect, and remove unwanted software from your computer are available on our Security Downloads resources page.

Note: Microsoft is not responsible for the quality, performance, or reliability of third-party tools.

Step 5: Surf and download more safely
The best defense against spyware and other unwanted software is not to download it in the first place. Here are a few helpful tips that can protect you from downloading software you don't want:

• Only download programs from Web sites you trust. If you're not sure whether to trust a program you are considering downloading, ask a knowledgeable friend or enter the name of the program into your favorite search engine to see if anyone else has reported that it contains spyware.

• Read all security warnings, license agreements, and privacy statements associated with any software you download.

• Never click "agree" or "OK" to close a window. Instead, click the red "x" in the corner of the window or press the Alt + F4 buttons on your keyboard to close a window.

• Be wary of popular "free" music and movie file-sharing programs, and be sure you clearly understand all of the software packaged with those programs.