Wikipedia
Syskey
Syskey is a utility that encrypts the hashed password information in a SAM database in a Windows system using a 128-bit RC4 encryption key that, by default, is stored in the Windows registry. Syskey can optionally be configured to require the user to enter the key at boot time as a startup password or load it on removable storage media (e.g., USB flash drive).
Syskey was introduced with Windows NT 4.0 SP3. It was meant to protect against offline password cracking attacks by preventing the possessor of an unauthorised copy of the SAM from extracting useful information from it. However, these days the feature is being misused by scammers to lock naïve victims out of their own computers and so coerce them into paying a ransom.