Would any of you be able to help with a similar issue?

I only installed one beta version of Office 2007: the "Beta 2" disk
which shipped with August 2006's PC Pro. So I guess that's pre
technical refresh. When the beta programme finished, we went back to
Office 2003.

I migrated my files, and my eldest son migrated his. Just now, my
youngest comes to me with a file he'd created & we can't open. Same
messages as above. File created in Jan 07, but the software hadn't
been updated since summer 06.

I can open it with Zip & examine the XML files - but he's got quite a
large table of numbers in there, so I'm not keen to go trying to
rebuild it from that!