Spyware and Adware have rapidly become the number one threat to computers.
Spyware and Adware include malware like "Trojans", Web Bugs, Advertiser Software, Dialers, Parasites, Scumware, Browser hijackers, Keyboard sniffers, Monitoring Software and more. These types of malware are often made by publishers that allow them to snoop on your browsing activity, see what you purchase and send you "pop-up" ads. They can slow down your PC, cause it to crash, and record your credit card numbers and passwords.
Norman Ad-Aware can help you combat such Scumware.