How do I turn off the following amazingly annnoying prompt?

This document is both encrypted and password protected. The office open xml
formats available in the 2007 release provide stronger encryption. Do you
want to increase the security of this document by converting it to an office
xml format?

I abhor Office 2007 (I've kicked Outlook to the curb and gone with
Thunderbird) but am forced to use it at work...but at home I use Office 2003
and my spreadsheets are in this format...but everytime I attempt to save them
under 2007 I get this prompt...

Surely there is a setting somewhere, or a registry hack that can get rid of
this...It particularly annoying as I have 4-6 spreadsheets that I use and I
have to go through this everytime I try to save them when they are updated...

Thank you...