If you want to protect your Windows XP systems from WanaCry/WanaCrypt ransomware install the following patch from Microsoft.
http://download.windowsupdate.com/d/csa/csa/secu/2017/02/windowsxp-kb4012598-x86-custom-enu_eceb7d5023bbb23c0dc633e46b9c2f14fa6ee9dd.exe
For Windows 7/8/10 just run Windows Update, it was patched back in March.
More info here: https://support.microsoft.com/en-us/help/4012598/title
Link to Microsoft Update Catalog for KB4012598 (server maybe overloaded): http://www.catalog.update.microsoft.com/Search.aspx?q=KB4012598
VirusTotal analysis (just in case): https://www.virustotal.com/en/file/3530b7890c22096693fd473d8c6455b9992ac4aa400e1b8ce14d0049234c489d/analysis/
SHA256: 3530b7890c22096693fd473d8c6455b9992ac4aa400e1b8ce14d0049234c489d MD5: 3AD11C9883051E5A5EEC5A000DC4C37C