What computers are vulnerable to hackers?

Historically speaking, any computer that is connected to a public network and running a Microsoft Windows operating system is more vulnerable to attack than any other operating system. Particularly, out of date, and unpatched versions of Windows are worse off than otherwise. This is due to several factors - Windows Operating Systems usually have more security vulnerabilities than other operating systems do, but mostly they are more vulnerable because of the popularity of Windows. There are many more people using Windows than other Operating Systems, so there is more incentive for hackers to target it.