While installing emacs for windows is easy (just unzip the archive and add the bin path to %PATH%), installing ispell is a bit tricky.
Before proceeding remember to add the HOME environment variable for your user. In this way emacs will search for .emacs, dictionaries and other files in there. In my case I used the C:\Documents and Settings\myuser as home. Otherwise it will search for files in C:\.
Now assuming you have emacs up and running, let’s install ispell.
Following what I’ve found in the Learning GNU Emacs book, let’s download the windows binary of ispell. Unzip the archive.
Move ispell.exe into the emacs’ bin directory and move english.hash into your %HOME%. Also copy this file as american.hash.
Now run emacs, open a buffer in text mode, type something in correct english and something not, and try ispell by M-x ispell-buffer. You should get ispell asking you what to do.
If everything is fine, let’s install the Italian dictionary. Obviously if you wont type anything in Italian, you wont need this feature.
Now copy the italian.words and italian.aff from the italian dictionary directory into the bin directory of the last ispell downloaded. Now from that directory give the command
C:\...\bin>BUILDHAS.EXE italian.words italian.aff italian.hash
and copy the italian.hash created file into your %HOME%.
For toggling automatically ispell into italian, see my .emacs file.