Read any pop-up messaging carefully before clicking on the window. Many times the message looks like an official warning to use some program or resolve a problem, but it can be an advertisement encouraging you to purchase some unnecessary program.
If you receive an official looking message or warning from a company, such as a bank or store where you have shopped, Do Not click any button to respond or verify receipt of the message. Call the company immediately, or open another browser and go to the company's official web site to contact their customer service on the web. Criminals might be trying to steal information using a web URL that is similar to the company's official web address.
Often, spyware and hijacking software are installed when you "consent" to a security warning window that displays over a webpage. By clicking on a malicious security warning screen, a script or control is integrated into the browsing system. The script or control changes the behavior of your browser to suit the needs of the aggressive advertiser.
To prevent this from happening, never click anywhere inside a security warning window that appears with webpages you do not trust. Close these windows by pressing the Alt and F4 keys together at the same time.