What is spyware?
Spyware programs make money for their publishers by reporting your Internet travels and sending you advertisements. Some also report your name, e-mail address, and other personal information.
How did I get it?
Many "free" programs come with spyware or adware. The program installer doesn't always describe every program being installed.
Are adware and spyware different?
Different people have different definitions of each. We feel any program designed to deliver ads or to get marketing information adware. Spyware is a subset of adware, focused on reporting personal information about you back to the companies who engineer it.
How do adware-removal tools work?
Most adware-removal tools act like antivirus tools. They maintain a library of spyware filenames and registry keys, and search for and remove them from your computer.
How are pop-ups related to adware?
Many adware and spyware programs deliver ads via browser pop-ups, even if you aren't online. Frequent browser pop-ups usually indicate adware has infected your computer.
What does a firewall do?
Firewalls detect and block incoming and outgoing Internet traffic. They can block spyware from sending your personal information to the Internet.
Is all adware bad?
Adware offers varying degrees of annoyance. Some adware merely delivers a small ad banner in a program's interface, such as the ad displayed in ICQ. Other types of adware launch pop-up browser windows over pages you're currently viewing. The worst kind of spyware gathers personal data about you and sends it to a central server.
How do I prevent adware from infecting my system?

Both Ad-aware and Spybot and Spysweeper come with immunizers that prevent known adware from installing. WinPatrol can also hold off suspicious program installations. When you install programs, make sure to read all the installer screens, especially the license agreement. Pest Patrol is a very good program, also.