Nearly 2 years ago, I wrote a blog article entitled Monad on Windows 2000. The article suggested that while you might be able to get Monad (as it was then known) to run on Windows 2000, it was probably a pointless exercise.
I notice from my logs that this earlier article is getting hits, so I thought I'd update the advice a bit. When I say "a bit" about the only bit is that it's PowerShell I'm talking about and not the beta (known back then as Monad). As I reported back in 2005, while you might be able to sort of install PowerShell under Windows 2000, the installation would rely on manual effort and probably wouldn't work anyway.
Of course, you can use PowerShell from your XP/Vista box to reach out and touch Windows 2000 servers across your network (e.g. using WMI). But as for running PowerShell on a Windows 2000 system - forget it!!