Ransomware! Did you read this article in the Times?
Ransomware is a particularly pernicious form of malware. It typically finds its way onto your computer when clicking on a link or an attachment from spam.
What happens next is your computer is locked and held for ransom — literally. You will be instructed to pay some amount of money to regain access. Even with virus scan software there is only one sure way to protect your computer, do regular backups. If this happens to you immediately unplug your external hard drives. If you have a current backup (you do backup, don’t you?), your system can be cleaned and restored without having to pay the ransom.