Spyware is software installed on your computer without your consent to monitor or control your computer use.
Clues that spyware is on a computer include a barrage of pop-up ads, a browser that takes you to sites you don't want, unexpected toolbars or icons on your computer screen, keys that don't work, random error messages, and sluggish performance when opening programs or saving files. To lower your risk of spyware infections:
Update your operating system and Web browser software, and set your browser security high enough to detect unauthorized downloads.
Use anti-virus and anti-spyware software, as well as a firewall, and update them all regularly.
Download free software only from sites you know and trust. Enticing free software downloads frequently bundle other software, including spyware.
Don't click on links inside pop-up windows.
Don't click on links in spam that claim to offer anti-spyware software; you may unintentionally be installing spyware.