The subject of this paper was the impact of computer crime on the security of computer systems and networks. It was aimed at small and medium-sized enterprises that undergo a restructuring phase to move to a new, electronic way of doing business or to improve their internal security policies related to the protection of confidential information. This paper was divided into five chapters. The first chapter is an introduction in which the topic of this paper and motivation for its selection were explained, followed by the second chapter and its theoretical approach to computer criminology, which presented the basic terminology related to computer crime, the aims and types of hacking attacks on computer systems and networks and provided an overview of impact and spread of this type of crime in the European Union and Republic of Croatia, followed by an explanation of the consequences that computer crime may have on the day-to-day business activities of the company. The third chapter introduced methods and tools to protect a company against attacks on information systems, all in order to help entrepreneurs better acquaint themselves with this issue and timely prepare for possible hacking attacks on their systems. In the fourth chapter, a survey was conducted using a questionnaire prepared by the National Committee of the International Chamber of Commerce in Croatia in cooperation with the Croatian Chamber of Economy aimed at investigating the degree of security of business information systems in order to help entrepreneurs carry out an internal audit on the company's ability to protect itself from threats to information security. Out of the 40 employees surveyed, the survey questionnaire was filled by 27 employees, representing 67.5% of the respondents. The results of the conducted survey were statistically processed and visually presented using graphs to make it clear in what limits our collected data were to be analytically interpreted. The fifth chapter is a conclusion summarizing the most important results and knowledge of this paper, followed by a list of literature, pictures and charts and questionnaire for participants of the survey.