Cryptolocker is what is known as a ransomware trojan virus that targets Windows machines. It is a highly sophisticated computer virus that encrypts files and then holds the files for ransom by requesting payments through Bitcoin or pre-paid cash payments.
Although the original virus has been identified and contained, there are new variants that claim to be the Cryptolocker virus and thus perform the same actions. Regardless, the best way to secure your files from any computer virus is to use a reputable anti-virus program and make sure it is updated daily. In addition, you should have multiple backups of your data either daily or weekly.
This will insure that if you are infected, you can safely restore the data from a previous backup.