xl2k and below came with an optional addin called AutoSave.xla. It could be set
to save every x minutes (user selectable). And it just saves the file at those
intervals.
xl2002+ comes with something called autorecovery. It's also optional, but if
the user turns it on, it saves a copy of that workbook in a special location
(also user selectable). If windows or excel crash, then the next time excel
opens, it notices that there's a file in that location. Excel prompts the user
to see if he/she wants to recover that file that was saved when excel/windows
crashed.
This autorecovery feature isn't used for the same purpose as AutoSave.
(If you have a copy of autosave.xla from a previous version, it'll work with
xl2002+, too.)
But since you don't want to have the user select options to turn on autosave,
you may find adding an ontime macro that runs every so often to your workbook's
project.
Chip Pearson explains how to use application.ontime:
http://www.cpearson.com/excel/ontime.htm