Leo says that there's a new virus going around that is actually really well written and difficult for antivirus to detect. If you get it, it will encrypt all of your data, and will require you to pay to get the encryption key. Leo wants to remind everyone to update not only your OS, but also other software such as Adobe Flash, Adobe Reader, and anything else you use that can be easily corrupted by malware.
ComputerWorld has an article on how to avoid getting CryptoLocker. "Cryptolocker comes in the door through social engineering. Usually the virus payload hides in an attachment to a phishing message, one purporting to be from a business copier like Xerox that is delivering a PDF of a scanned image, from a major delivery service like UPS or FedEx offering tracking information, or from a bank letter confirming a wire or money transfer."
If you do get stuck with this, you really have only one option: Pay up and get your data back or just format your hard drive and reinstall windows. Either way, you should do the latter.