You could use an OnTime event.
Chip Pearson has lots of instructions at:
http://www.cpearson.com/excel/ontime.htm
and...
Jan Karel Pieterse wrote an addin (works in any version) called AutoSafe (note
spelling).
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.