Be aware there is a new virus called Cryptolocker that usually comes in through an executable attached to an email, usually about a parcel delivery. Do not open the attachment. It can come in other ways too, through a botnet if you’re already infected with another virus.
The virus will encrypt your files and unless you pay the $300 to the criminal virus creators.
Here is a video showing the virus in action by Sophos.
Here is an excellent article about the infection. Bleeping Computer (always my heros) does a have a partial solution.
An action to protect yourself: please make a backup or image of your computer now and then remove the external hard drive. Make sure you then detach the hard drive from your computer. Ducktoes can do this for you if you bring a computer in to our Calgary Computer Repair lab.