What version of excel do you have?

In xl2k and earlier, there's an autosave addin you can turn on (tools|Addins).
If you don't see it there, have your distribution CD available.

In fact, that addin will work with xl2002 and xl2003, too!

If you're using xl2002+, you can turn on autorecover--but this is different from
AutoSave.

The workbook is saved as a different name/folder every xx minutes (user
selectable).

If excel/windows crashes, the next time you start excel, it'll notice this file
and prompt you to find out if you want to use it.

If excel closes normally, this file is deleted (never to be heard from again!).

Another option:
You can use an addin from Jan Karel Pieterse called AutoSafe.

It doesn't overwrite the existing workbook when it saves.  It saves to a user
selectable folder.  And when it's done, it either deletes these backups (or puts
them in the recycle bin).  And the user can always restore the backups from the
recycle bin.

http://www.jkp-ads.com/Download.htm
(look for AutoSafe.zip, not autosafeVBE.zip, for your purposes.)

Jan Karel's version will know if there was a crash and prompt you to open the
last version it saved.