See info pages on ispell.
Current favorites appear to be aspell and hunspell.
Point to the ispell replacement:
(setq-default ispell-program-name "hunspell")
You might already have the required dictionary files in your browser or openoffice directory tree.
Windows XP difficulties
Links to precompiled binaries are dead at time of writing.
I got RPMs from openSUSE, but hunspell.exe aborted after not finding a dll file. After that dll was added, another dll was not found. I gave up after several iterations. This was with mingw32-hunspell-1.3.2-1.98.noarch.rpm and later mingw32-hunspell-1.3.2-1.99.noarch.rpm (and associated lang and tools RPMs).