Spyware is software that is often deceptively loaded on your PC without notifying you that it will load and what it will do. Usually this type of software collects personal information (including web sites visited, user names, passwords, etc.) without the knowledge of the user. This information is collected by remote computers without the users knowledge and can be used to perform illegal activities and/or subject the user to unwanted communications from companies that they have no desire to interact with. Spyware is related to adware, however, it is often embedded in software downloads for free software without informing the user that it will be included in the download/installation process.
Spyware is software that collects personal information from you without first letting you know what it's doing and without letting you decide whether this is OK or not. The information spyware collects can range from all the Web sites you visit to more sensitive information like usernames and passwords. You might be the target of spyware if you download music from file-sharing programs, free games from sites you don't trust, or other software programs from an unknown source.
Spyware is often associated with software that displays advertisements, called adware. Some advertisers may covertly install adware on your system and generate a stream of unsolicited advertisements that can clutter your desktop and affect your productivity. The advertisements may also contain pornographic or other material that you might find inappropriate. The extra processing required to track you or to display advertisements can tax your computer and hurt your system performance.
The key is whether or not you (or another user of your computer) have been properly notified of what the software will do and that you have provided consent to have that software installed on your computer. In other words, is the software being deceptive in what it does or how it gets onto your computer?
Spyware and unauthorized adware are two examples of "deceptive" software. Deceptive software includes programs which take over your home page or search page without first getting your permission. There are a number of ways deceptive software can get on your system. A common trick is to covertly install the software during the installation of other software you want such as a music or video file sharing program.
Whenever you are installing something on your computer, make sure you carefully read all disclosures, including the license agreement and privacy statement. Sometimes the inclusion of adware in a given software installation is documented, but it may appear at the end of a license agreement or privacy statement.
Sometimes deceptive software gets silently installed on your system without any warning at all. If you use Internet Explorer as your Web browser, this can happen if your Internet Explorer security setting is set to its lowest value. Make sure to keep this setting at the medium level or higher. Doing so will help you control what is being installed on your computer. (We'll discuss this more in a moment.)
Have you ever had an experience where you were repeatedly asked to accept a download even after you said "no"? Creators of deceptive software often use such tricks to get you to load their software. If this happens to you, do not click "yes". Instead, try to close the Web page that first asked you to accept the download by hitting the "X" in the corner of the window. Alternatively, quit Internet Explorer and restart it to begin browsing the Internet again. If you visit a Web page that continually displays these tricky pop-up windows, that Web site may not be worthy of your trust.
Microsoft provides useful information about spyware/adware.