Windows 2000 is a slow booting OS period.

My PIII 1GHz desktop boots in 47 seconds (timed it once) under Windows 2000 Professional (no Caviar drive here). It did boot faster under XP Professional when I tested it, however. As for other OSs - I would consider some form of Linux, but it's rather intimidating, and I think most of my software wouldn't run under it, anyway.