FVWM has a Win9x theme that looks very nice, although it is clunky (I like to use a "clean" FVWM)
http://fvwm-themes.sourceforge.net/ (then choose Redmond98 as your theme)

Gnome and KDE both have "XP" themes for them...
IIRC this is for Gnome: http://art.gnome.org/themes/gtk2/637/

IIRC this is for KDE:
http://www.xptheme.info/resources_view.cfm/hurl/specific=90/VisualStyles=/rnam=KDE%20XP%20Beta%201/

Also can be found (for both)at:
http://themes.freshmeat.net/projects/experience/?branch_id=53380&release_id=201881

If anyone has better themes please post a link...Thanks!