There is a new strain of malware going around called Cryptolocker. This malware infects your system from a website ad, downloaded file, or e-mail attached. Once on your system, it encrypts the files on your computer so that you are no longer able to open them. It then begins generating pop-up messages demanding that you pay a ransom in order to be able to access your files again.
This malware is particularly aggressive and destructive. The only way to recover files on an infected system it is to restore them from a backup if one exists. Anti-virus software vendors have been working on a solution, but due to the nature of the file encryption method used, obtaining the decryption key needed to reverse the damage is proving extremely difficult.
You can read more about this malware outbreak, and recommended safety steps, here:
Also, we want to remind users to make sure that backups are in place for important files (ideally to the cloud but optionally to a USB drive/stick or local server). Please be extremely careful when browsing the web and practice safe computing techniques. In particular, do not open email attachments such as .zip, .pdf, or .exe files if you are not expecting them or should you receive them from unknown sources.