We tried Windows 2000 Professional for the DEC Alpha for a GIS system in the late 90s. Suddenly made the $5000 PCs that could run it seem cheap.

Unfortunately, NT for Alpha only ran in a 32-bit address space.

"The 64-bit versions of Windows NT were originally intended to run on Itanium and DEC Alpha; the latter was used internally at Microsoft during early development of 64-bit Windows. This continued for some time after Microsoft publicly announced that it was cancelling plans to ship 64-bit Windows for Alpha. Because of this, Alpha versions of Windows NT are 32-bit only."

https://en.wikipedia.org/wiki/Windows_NT#64-bit_platforms

Alpha support was removed in one of the later NT5 betas right? Makes sense that it would've been late 90s then, before it was renamed Windows 2000 for release.