The spread of broadband Internet has resulted in the increase of spyware and adware. This study highlights their damaging effects and proposes a model that captures defensive measures adopted by Internet users. Specifically, our model indicates that knowledge has a positive impact on self-efficacy that, in turn, is presumed to trigger technical defensive measures. Moreover, concerns for privacy and previous experience with spyware and adware are likely to evoke both tactical and technical defense measures. Data collected by the PEW Internet and American Life research project are utilized to test the proposed model and the findings are discussed.