A Longhorn simulator isn't a full operating system. Instead, it is typically a high-fidelity recreation of the Longhorn user interface (UI) built using web technologies (HTML/JavaScript), Flash (in the older days), or standalone software like Visual Basic.

For the purists, "simulating" Longhorn means running the actual leaked ISOs (like Build 4015 or 4074) in a Virtual Machine (VM) like VMware or VirtualBox. This is the closest you can get to the real thing, though it requires hunting down old drivers to get the graphics working correctly. The Legacy of the Simulator Community

If you’re looking to dive into the "Plex" aesthetic, you have a few different paths: 1. Web-Based Simulators

Before it was a clunky addition in Vista, the Longhorn sidebar was envisioned as a central hub for communication and live data.