At almost every Web page Chris Tooley visits on the Internet, the first image he sees is an advertisement.
Animated characters dance across some ads.
Other ads, called ''pop-ups,'' spawn small windows on the screen that block part of the underlying Web page. Working with an Internet browser, the programs prevent ads from appearing on a Web page but generally display all the other text and images. Sometimes, the software leaves just a blank space where the ad was. Other times, there's no trace where it would have been. Endorsed by some privacy advocates, ad-blockers also often stop ''cookies,'' tiny files that advertisers use to invisibly collect personal details about consumers, such as how often they visit a site and which pages they read.