Adware 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 information on web sites you have visited, provides additional popup windows with copies of your web browser running with unsolicited advertisements (hence the term adware) or automatically loads numerous web browser windows when you open your web browser (i.e., Internet Explorer) or potentially changes your list of Favorite web locations to visit or your home page of your web browser. The objective is to target unsolicited advertisements to the PC user and tempt the user to visit the unsolicited site and/or purchase unsolicited products. A more deceptive adware software tool may try to trick the user into supplying personal information (including user names, passwords, account names/numbers, banking information, etc.). These latter, more malicious adware and mass mailing techniques are referred to as Phishing.
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.