Accessing a locked system is always a challenge. While you might be tempted to pull the plug and image the disk, you could miss a lot of valuable evidence if you do. Full-disk encryption, EFS-encrypted files and folders and everything protected with DPAPI (including the passwords stored in most modern Web browsers) are just a few obstacles to mention. Recovering the original Windows logon is a must to access the full set of data, while resetting the logon password may help unlock working accounts in emergencies.
The Encrypting File System (EFS) was first introduced in Windows 2000 and, as Microsoft claims, is an excellent encryption system with no back door.