Your computer is acting slower than usual: One of the most commonly noticed symptoms of spyware is that your computer may responding much slower than usual.

Your Internet connection appears slower than usual: Spyware can use significant computer memory resources. Multiple instances of spyware running at the same time can create “memory leaks,” which can give you the impression that your Internet connection is much slower than normal.

Advertisement pop-ups appear when you're not browsing the Web: Spyware can also serve you pop-up advertisements that appear while you are not surfing the Web--even if you don’t have a Web browser open.

Your homepage has changed, or keeps changing: Sometimes spyware will change your default homepage, or render your computer incapable of changing your homepage again.

Redirecting Web sites: Spyware can cause your browser to automatically redirect to different Web sites. This symptom is especially common with Internet search engines. Search tool bars appear in your browser: You may notice that search bars appear directly in your browser. These bars, also known as plug-ins, can sometimes be installed by spyware programs.

Spyware software appears in menus: Another indication that spyware has been installed on your computer is when suspicious programs and folders appear in your Start menu, startup menu, or system tray.

Unintentional Download -- You may intentionally download and install spyware or adware without realizing it has an unrelated, secondary purpose. For example, you may be told a program will keep your computer clock synchronized with the atomic clock. If it is spyware or adware, the program may also serve send information about your browsing habits or display advertising pop-ups whenever you are online.

Software Bundle -- Spyware and adware may be included (bundled) with programs you download and install. For example, many file-sharing programs require you install additional, unrelated programs that may be considered spyware or adware.

Unintentional Install -- You may voluntarily but unintentionally install spyware or adware while surfing the Web. Often times, spyware or adware appears to be corrective software or a special plug-in that will help you view a site. Or, for example, you may be told a program will keep your computer clock synchronized with the atomic clock. If you don't know the source of the download or aren't sure what a program does, you can generally learn more by searching on the name of that program in Yahoo! Search.

Other Users -- Someone else who uses your computer may also install spyware or adware on your computer. They may do so unintentionally in any of the ways mentioned above, or they may maliciously install a program with keystroke logging, to capture passwords or other personal information.

Computer Virus or Worm -- A computer virus or worm can also install spyware that could allow someone to remotely access to your computer, turn your computer into a pornography or spam server, log keystrokes to steal passwords or credit cards, or perform other malicious acts.

Automatic Installation -- Some unscrupulous companies use advertisements to automatically begin the installation process for their programs. By default, most browsers will prompt you with a security warning to either continue or stop the installation process. Typically, these warnings don't offer a description of the program or tell you what it will do.