Adware and spyware are the computer pest of the modern world. There are many occasions when somebody who uses a computer can get infected with such programs, and getting rid of them is many times a real pain.
Most of the times, adware is voluntarily installed by the computer owner himself: there are software companies which deliver a free version of some programs, for everybody to download and use, but bundled with adware. Each time the program is launched, the user sees a commercial message. This is the price to pay if you want to use software without paying real money for it.
If you want to keep your computer clean from adware and spyware, the best method is not to connect it to the internet at all. As this is less probable to happen, at least you can avoid installing any freebies you find on various sites, and use only software coming from trusted places and sources.