There's a particularly evil piece of Malware doing the rounds at the moment, called CryptoLocker.
On infection, it encrypts files both locally and on mapped network drives with RSA 2048-bit encryption. The only way to regain access is to pay the people behind it $300, which a lot of people are actually doing at the moment.
The most common form of infection at the moment is via attachments on unsolicited emails, so be careful what you click on. There's some good technical information on the Bleeping Computer website, along with some steps you can take at a Group Policy level if you're responsible for a corporate network.
Yet another reason why a) you should take regular backups of anything important, and b) why "RAID" is not "backup." Also of note is it'll attack mapped network drives, so that backup you've painstakingly created on your permanently connected USB hard disk / NAS? Worthless.