There is a new and particularly dangerous virus making the rounds recently. It is called Cryptolocker. Instead of damaging your operating system or exporting your personal or firm data like many viruses and other forms of malware, Cryptolocker finds and encyrpts your documents and files so that you cannot open them. Without the encryption key to unlock them, they are useless to you.
The hackers behind Cryptolocker demand that you promptly pay $300 for the decryption key to unlock your own documents and files. If you fail to make the payment in time, the decryption key will be destroyed and access to your files will be lost forever. Unless you have a backup of these files that was not accessible to the Cryptolocker ransomware (a good argument for on-line backup with a versioning feature), paying the $300 ransom - and to doing it before the decryption key is destroyed - may be the only way to decrypt your files. We don't recommend paying a ransom to cyberterrorists, so make sure your backup system is adequate and functioning properly.
It isn't clear that antivirus and antimalware protection programs will stop Crytolocker from invading your computer. It is more likely that Cryptolocker is downloaded when users click on a link in an email or on a web site. Great caution is urged when clicking on any link sent to you via email, even if it appears to come from a trusted source such as your bank or a service like PayPal. The prudent approach is never to click on a link that you are not expressly expecting to receive, even if you think you know the sender.
For more information on Cryptolocker and the significant danger it presents, see this post on the Malwarebytes blog.