To avoid infecting your system, please remember to never read or open attachments in emails where you do not recognize the sender, and/or are not expecting an attachment from the sender. An example of the sort of risks associated with opening unknown emails/attachments is linked below:
Ars Technica: Ransomware comes of age with unbreakable crypto, anonymous payments.
Also, tips for avoiding opening the wrong emails are here: http://www.pcworld.com/article/116586/article.html