Logged in, and there's some notification asking me to backup the key because it is encrypting my file system. I didn't ask it to do that, and I wasn't even logged in as admin before the reboot.

Vista relies primarily on AES (Advanced Encryption System), using it as the encryption algorithm both EFS (Encrypted File System) and BitLocker drive encryption.

How to Encrypt and Decrypt a File. Encrypting files on your computer helps to secure your data from unauthorized access. Microsoft Windows features the native ability to encrypt files and folders ...

The lab-developed ransomware strain relies on the Encrypting File System (EFS) component in Microsoft's operating system and can run undetected by some antivirus software.

In fact, the Encrypting File System (EFS) has been around since way back in Windows 2000, as a feature built right into the NT file system (NTFS), on which Vista, XP, and Win2K have been based.