Not sure when it happened or if it's always like this.

Upgraded from Office 2003 to 2007. Set everyone up to save to XLS by
default, since most people can't use or view the XLSX crap.

So, one customer mails us an XLSX file. This lady tries to open it and it
opens blank. So, we're sitting at the Excel screen with a light blue
background as if no file was opened.

Yet, someone else was able to open the same file just fine.

Any ideas?