Malware of the family Trojan-Ransom.Win32.Xorist, Trojan-Ransom.MSIL.Vandev is designed for unauthorized modification of data on a victim computer. It makes computers uncontrollable or blocks its normal performance. After taking the data as a “hostage” (blocking it), a ransom is demanded from the user.
Kaspersky’s XoristDecryptor utility is available here – http://support.kaspersky.com/us/viruses/disinfection/2911#block2