videopool by djpool.net
part of the remix.network

While actual beta builds of Windows Longhorn (which later became Windows Vista) primarily used , the specific "Longhorn" sound packs available for download as .wav files have a distinct reputation online: User Reviews & Historical Context

While Windows Vista eventually settled on a soaring, choir-like orchestral swell composed by Robert Fripp, the earlier Longhorn startup sounds were different. Many builds featured a shorter, synthesized chord that shimmered. It was less about announcing "You have arrived" and more about waking the computer up gently. It was a blend of digital chimes that felt cold yet inviting—the sound of a machine thinking.

Windows Longhorn remains the most famous "what if" in operating system history. Developed between 2001 and 2004 before being famously "reset" into what became Windows Vista, Longhorn promised a radical departure from Windows XP. Part of that visionary experience was a completely unique, ethereal soundscape that enthusiasts still hunt for today.

If you want, I can:

: For a quick trip down memory lane without manual installation, the Collection of Windows Startup Sounds