Ahh, following up your own questions is always fun. I noted a thread on photo.net talking about running ACS Classic on Windows 2000 at http://photo.net/bboard/q-and-a-fetch-msg.tcl?msg_id=000spp&topic_id=web%2fdb&topic=
The discussion points to http://dev.arsdigita.com/doc/win2k-installation.html, which says that it is doable, but not recommended for production use.