WannaCry wouldn't take off if everyone updated Windows.
For those who use operating systems that are not removed from technical support, it is enough to simply update the operating system or install one update. Read more here https://technet.microsoft.com/en-us/library/security/ms17-010.aspx?f=255&MSPPError=-2147217396
For those who use outdated operating systems:
- Windows XP SP2 x64
- Windows XP SP3 x86
- Windows XP Embedded SP3 x86
- Windows 8 x86
- Windows 8 x64
- Windows Server 2003 SP2 x86
- Windows Server 2003 SP2 x64 for Windows
It is also strongly recommended to perform a virus scan with any antivirus.