You could do it if you wanted to custom draw all the elements of your
forms, but that would be a HUGE undertaking. Basically, you have to have XP
for the XP look and feel.
As Nicholas said, you could paint all your UI by hand. Or you could buy one
of those third party Windows Form Controls libraries where you would find
XP style buttons...
However, i don't think that this is a great idea. I don't know about the
others but i personnaly hate applications that try to look nicer by having
a fully cusomized UI that looks nothing like any other application. In my
opinion there should be consistency in the look and feel of all
applications under a particular OS or it becomes quickly a total mess. If
users are not happy with the look of Windows 2000, then they can use one of
those desktop skining software. But this must be their choice, not yours.