This paper can be divided in two main parts: one describes instructions and advices for computer safety which business systems need to implement if them want to ensure the security of their operating system (regardless of which OS they use) and basics of cryptography and cryptographic attacks, along with methods of defence for them, with special emphasis on cryptographic methods used by Windows (LM, NTLM). In other part I have given an overview of security principles and protection mechanisms of systems Windows 7, Windows 8 and Windows 10: how each of those mechanisms work and which type of attack they prevent or protect from. The reason I chose to write about secuirty and protection of Wndows OS is because Windows operating systems make the majority of operating systems used in the business domain. Their is one chapter dedicated to solving the assignment of this master thesis: configuration of Windows Firewall (firewall in one of basic tools for protecting OS and computers) and creation and implementation of Windows Firewall rules and policies. I used both textual and image description in chapter which makes it, basically, a tutorial.