Why Do Ads Pop Up on My Computer?
There are a number of reasons why ads may pop up on your computer, ranging from benign (overuse of advertising on a website) to malicious (adware infecting your computer). Learn how to differentiate between them and what to do about adware!
There are really two times when you'll see ads appear on your computer. If you're browsing the web and an ad pops up, it's most likely from the website that you're browsing. They'll either show you a popup (which shows up right then) or a popunder (which shows up after you've closed your browser, since it's hidden by your browser until you close it). While annoying, these ads do not do anything other than want you to buy things.
On the other hand, if you're doing something non-web related (such as word processing) or viewing a website which you know does not have pop-up advertising (such as Google), then your computer is likely to be infected by adware. Adware is a type of malicious program that shows up on your computer to display advertising.
They sometimes try to masquerade as legitimate programs that warn you of your computer being infected, but generally all that means is that they're trying to sell you some sort of hokey ad removal progarm (that will generally just install more ads on your system, not remove any of them).
Adware can be exceptionally tedious to remove. It's designed from the ground up to be hard to remove just to thwart the efforts of the owners of the computers it has infected. The best thing to do here would be to grab an anti-adware program and let it take care of wiping away all the adware on your computer (and making sure that it does not get infected again).