Adware is, by definition, something reasonable people don't want on their computers. That's why adware can't just come out and ask people to install it. Often, the computer owner is completely unaware of it being installed. But not always

When adware doesn't want to sneak in through an open window, it will try to trick you into letting it in through the front door. Don't think you could be tricked? Don't be so sure until you've checked out these most common ways people have been tricked into allowing malware to be installed on their machines.