CryptoLocker Ransomware on Rampage
An increasing number of infections have been reported from the relatively new CryptoLocker malware, which encrypts files and holds them for ransom. Unlike previous ransomware, CryptoLocker makes its targeted files legitimately unrecoverable and also appears to honour ransoms by decrypting the files when paid (at least for now).
Naked Security has a pretty accessible overview of what CryptoLocker does and how to avoid infection. The primary vector appears to be email attachments, so please be particularly vigilant and do not open attachments unless you are certain of their contents.