PGPDisk
PGP Virtual Disk is a disk encryption system that allows one to create a virtual encrypted disk within a file.
Older versions for Windows NT were freeware. These are still available for download, but no longer maintained. Today, PGP Virtual Disk is available as part of the PGP Desktop product family, running on Windows 2000/XP/Vista, and Mac OS X.
The system requires troubleshooting when operating within Windows 10 and 11 due to their Device Guard.