First, get ispell for win32 from this page, extract it and place it to some place you are comfortable with, modify ispell.ini in the bin directory to point to where the dictionary files are. Although it runs on windows, the path should be forward-slash, not backslash as found on windows machines.
I use the dictionary file found on this page, which the author (Suzuki Nobue) said on the page that he used dictionary file found in FreeBSD's dict distribution, with tools that came with win32 ispell to convert the dictionary file from dict format into ispell-recognizable format.
To use ispell with windows, you can either modify the XEmacs init.el files to tell where ispell is located, or simply place the path to ispell in windows's PATH so when XEmacs calls ispell.exe, the executable is found. (I am lazy and not at all know how to program in lisp, so I choose the latter than former)
[1] xyzzy software download page http://www.jsdlab.co.jp/~kamei
[2] ispell on xyzzy (Suzuki Nobue) http://www2.odn.ne.jp/suzukinobue/pc/ispell.html