Spyware is software which collects demographic and usage information from your computer. Some types of spyware can make changes to your computer, causing it to run slower or even to crash. Spyware programs can also change your Web browser’s home page or add toolbars that you may not need or want.
Other programs called browser hijackers can be used to direct you to rogue search engines or they can display a message that a requested page is no longer available. Adware applications install advertising components on your computer system that download ads and waste your system resources. Many of these applications or components can be running on your system even if you are not using the software that initially installed them.