The free version is fully functional; you only need to run the updater manually. In addition HitmanPro.Alert has the ability to block the encryption process itself. Use both since one is static and the active. See here:
CryptoWall 3 and CTB-Locker defeated by HitmanPro.Alert - YouTube
As others have already stated a regular data backup is the only 100% guarantee that your data is safe, but using these utilities that are specifically designed to block crypto-ransomware will potentially save you a lot of grief and hassle.
I would absolutely not trust Windows defender to protect against crypto-ransomware.