Always be careful what e-mail attachments you click on, make frequent backups of your most important files and keep your system and Anti-Virus up-to-date!
Story on Cryptowall virus:
http://arstechnica.com/security/2015/06/fbi-says-crypto-ransomware-has-raked-in-18-million-for-cybercriminals/
Microsoft: How to keep your system updated
General advice on keeping your computer safe.