Ransomware is a growing threat to computer users, who can suddenly find they're unable to open or use their files when their machines are infected. The malicious software can attack any user — an individual, small business, Fortune 500 company or a government agency. A program called CryptoLocker appeared several years ago, and made files like word processing documents and photos inaccessible to computer users unless they paid a ransom, usually $500 to $700. A.