TrendMicro have release an informative whitepaper on how to identify Fake Antivirus programs, often refered to as FakeAV. These programs generally pop up on your computer screen, and inform you that your computer has been infected and then offer to clean it out for you. Click on the program will inadvertently install the program, which in turn, installs a whole bunch of nasties.
You can read all about it here – http://solutionfile.trendmicro.com/solutionfile/1055338/EN/Unmasking%20Fake%20AV%20v3.pdf
…and hopefully educate yourself on how not to get infected by these apps.