I need step by step instructions to convert an iSpell .hash file into
plain ASCII text, from Windows and/or a DOS command prompt (not
Linux).  I'll download and use any binary I'm pointed to, but I don't
want to compile binaries from source to do this (that's usually a
nightmare in Windows).

I've got iSpell installed - there are a number of .EXEs with it -
perhaps one of them can do this, but I don't know the syntax.  In my
ISpell directory, I see
add.bat
buildhash.exe
icombine.exe
ijoin.exe
ispell.exe
sq.exe
unsq.exe

After some study of ispell sources I've written hash2dict program.
Binary complied for win32 is available at
http://www.webfile.ru/get/1823755217/hash2dict.zip

Run it like

hash2dict.exe my.hash

and it will extract all words from my.hash into dict.txt text file.

That file does not appear to be available.  When I put that url in my
browser, I get a site that's all in Russian, with nothing that I can
see to download (though I can't speak Russian, so perhaps it's there
but hard to find.

I also looked at the source files... I created the application
(including the source code), compiled with win32. Get the zip at:

http://www.adventuresunion.net/stan/hashdump.zip

To extract the wordlist run the following in command prompt:

hashdump foo.hash

The words will be printed to the screen. If you want to put them in the file run:

hashdump foo.hash > myoutput.txt

Now the words are not sorted. If you would like a sorted output, you
will need to pipe the output through the sort command, like this:

hashdump foo.hash | sort > mysortedoutput.txt 

OR

hashdump foo.hash > unsorted.txt
type unsorted.txt | sort > mysourtedoutput.txt

Let me know if it works!

Sageres's response seems to work.  Though I'm still puzzled by what
all the extra data in the last 2/3 of a .hash file is.  For instance,
the italian dictionary here
(http://members.xoom.virgilio.it/trasforma/ispell/), claims to be 185K
words, but I only get 45K words (and I had to disable the early exit
on consecutive 0s to get that).  Anyways, this is helpful and worth
crediting as a correct answer, so go ahead and repost it that way.

Hm... I could not quite determine exactly how to tell the size of the
hash table. When I counted the number of strings, it was always bigger
then hashsize value.

If I find something to add to it I will add more info...