The Cryptolocker malware has been spreading across the US. It infects your computer and encrypts your data files with a key that only the attackers have. They demand that you pay them to get the key and unlock your files. This threat can actually result in data loss, so BE AWARE! Do not open any e-mail attachments that you were not expecting. The e-mails appear to come from Intuit (QuickBooks), FedEx, UPS, etc.
To prevent this sort of thing, do the following:
1. Back up your data on a regular basis. You can back up to a USB hard drive, or to “the cloud”. Business networks should have an automated backup solution.
2. If you don’t have an anti-virus/anti-malware program, get one. Once you have one, make sure it stays up-to-date. Try Trend Micro, Symantec (Norton for home users), etc.
3. Use a spam filter. This may be part of your anti-virus solution, or it could be an outside service.
4. Regularly check that your backups are ACTUALLY working and your anti-virus is working properly.
More detail about Crytolocker can be found here: http://arstechnica.com/security/2013/10/youre-infected-if-you-want-to-see-your-data-again-pay-us-300-in-bitcoins/ All of the anti-virus vendors have information as well.