Emacs has support for spell checking on demand (
ispell) and as
your type (
flyspell). Both packages depend on a copy of
ispell 3.2 or a compatible spell-checking program.
GNU Aspell is a popular choice these days, Windows installers are
available from the official site.
Once installed, you will need to configure
to tell ispell and flyspell to use aspell as a replacement for
ispell. You can include the full path to the aspell binary, which
means you do not need to add its installation directory to the PATH.