Spyware or adware are two members of the malware (malicious software) family that plague a lot of operating systems these days.
Symptoms can vary from spyware to adware and include everything from a slowed computer to added icons and changed start pages on your browser.
Below is a link index containing useful sites to help you learn more about these programs and how to protect yourself from an infection. Many free download sites place adware within their software packages so it is important that you know what to look for before you download files or open attachments. The first link to the Federal Trade Commission is a good place to begin.