Adware and spyware mainly run on your computer with out your knowledge so it is sometimes hard to know if you have an adware or spyware program running on your computer until it causes a problem.
Some of the common problems caused by adware and spyware are reduced performance of your computer, random pop-up advertisements, the addition of new tool bars in your web browser, and (as mentioned above) a change in your current homepage.
When problems such as these show up on a computer, the first thing that an IT Professional thinks of is adware and spyware, which is the most common cause of these issues. Unfortunately, the issues caused by adware and spyware can become so severe that many users will need the assistance of a qualified IT Professional in order to resolve them.
Less common but more dangerous problems cause by adware and spyware are corruption of data files, identity theft, and viruses. All of these problems can be very costly and are best avoided. These less common, but much more dangerous, adware and spyware programs are the main reason that prevention is so important.