I've checked this out and it is a genuine threat, affecting PC but not Mac. Just want to warn everyone to be on your guard.

It has happened to a friend of ours and he has just lost all his photos and many other important things.

He had recently done some purchases online, as we all do this time of year, this left his guard down so when an email arrived from UPS saying his parcel had been despatched and to click on the attachment for the tracking number he did not take the usual precautions to check it was definitely addressed to him by name.

He then got a message saying that all his important files had been locked and for them to be released it would cost $700. If you pay they are released, which is awful, how do these people get away with it!!

More here on Snopes: snopes.com: CryptoLocker