You're referring to , which is an early pre-beta build from the M3 (Milestone 3) development phase, compiled around May 2008 . This build is historically notable because it still carries a strong Windows Vista "Longhorn" aesthetic while introducing features that would eventually define Windows 7.
If you want to try it in a VM, expect frequent explorer restarts and missing drivers. It’s a fascinating time capsule, not a production OS. windows 7 build 6730