While I do not see any advantage to go from WinXP to Win2k, perhaps you do.
To downgrade I would suggest that you get your needed files off, then reformat your hard drive. Installing Win2k then should not give you any problem with going to an older version of OS.
I have a toshiba m200 running windows xp. I want to downgrade from xp to windows 2000, but when i boot up my computer and insert the cd, it says "setup is disabled because it is an older version of windows". Of course those are not the exact words it said but, you get the idea. Is there any way to get around this?