Spyware is one of the top threats and attacks becoming widespread and dangerous in information and computer security. As a result of malicious usage of spyware, all sort of home and corporative users trying to make use of computer technologies are exposed to severe losses. We review the methods that spyware uses to infect computer systems. The methods are supported by concrete examples. The preventive measures against spyware are presented in both user and system administrator level. Comprehending spyware infection methods makes users wide-awake and following and applying the measures presented ensures the effective protection.