When your surfing the web, do you get random popups? If so, then you probably have hidden 'adware' installed on your computer.
Adware is similar to computer viruses and spyware, in that it hides itself within your computer. But the purpose of adware is to display adverts on your computer. Usually within a popup,
Some new adware can now actually change the ads which are being displayed from a web page with its own ads.
The easiest way to check and remove any adware programs in your computer is to install an anti-virus.
Though adware hides itself in your computer, it often doesnt do its own job very well and causes problems with your computer system. Certain web pages may not load or work properly.
A lot of anti-virus software only removes adware that it considers particularly harmful. Though this is starting to change.
Recommended anti-adware software
Ad-Aware SE Personal Edition