While some adware can consist of merely harmless advertisements, other adware can be much more destructive. Abusive adware can cause: excessive pop-up ads; serious decrease in system performance; the manipulation of Web sites; the delivery of spyware and other malware; and more. Many freeware and shareware programs include adware, though most users don’t know about it until after they download. And once adware is downloaded to a system, it can be nearly impossible to get rid of.