In todays world, every machine on the internet is a potential target for attackers. Whether they are looking for a drone to use in a denial of service attack or to steal personal information from your system they are a threat you cannot ignore.
Unfortunately many of todays operating systems were developed before use of the internet was as widespread as it is today and little attention was paid to security. This is thankfully changing, even Microsoft is beginning to acknowledge that security is an important issue even for a home computer and are designing their new products accordingly.
The steps required to secure your operating system differ depending on which OS you use and which version of that OS you have installed.