Cryptolocker is a fairly nasty virus which is well documented. The most useful FAQ on the virus is on Bleeping Computer HERE
We *HIGHLY RECOMMEND* that you look at the how to prevent section HERE which tells you how you can stop this (and other) malware from running. You can do this on a per-computer basis, or via Group Policy.
A company called FoolishIT (make sure you read that as Foolish IT 🙂 ) – have created a utility called CryptoPrevent which is ideal for small businesses or home users who need to make the changes quickly.
We STRONGLY SUGGEST you download CryptoPrevent and run it (as administrator) and suggest you apply all the options on each of your computers.