If you are home or small business user running in a non-enterprise environment where comprehensive Anti-Virus / Anti-Malware protection, advanced file services and encrypted backups are unavailable, you can protect your personal computers with light and free Anti Ransomeware tools by Malwarebytes or BitDefender (among others).
Malwarebytes Anti-Ransomeware - running on the background consumes around 14 MB of memory. Product is still in Beta but being actively developed. Download link: https://forums.malwarebytes.org/topic/177751-introducing-malwarebytes-anti-ransomware-beta/
Bitdefender Anti-Ransomeware - running on the background consumes around 5 MB of memory after initial startup. Very light signature with the pleasant and simple interface. Download link: https://labs.bitdefender.com/2016/03/combination-crypto-ransomware-vaccine-released/
If you are running fully featured commercial protection software, this feature may already be integrated. Check with your vendor tech support or product support forums.
Always treat your email attachments with healthy suspicion - especially files with EXE , ZIP, TAR and even DOC / XLS / PDF extensions as some documents may be programmed to run nefarious scripts and macros . If you are not sure what to do, please, ask someone who does.
Update for server administrators: Enabling "Shadow Copies" on your server drives may not guarantee recovery of encrypted documents using "Restore previous versions..." option. Some ransomware strains now manage to delete all cached checkpoints on affected drives where the only remaining recovery option is to have reliable data backup.