We have a mix of Office/Excel 2003 & 2007. Until yesterday, this has not
been a problem, as 2007 users save files so as "97-2003 workbook".
Yesterday morning, this all was fine. In the afternoon, problems started
happening and today it has escalated. None of my XP Pro with Office 2003 are
able to open a file that they all need that is created and modified by a
Vista Excel 2007 user. When the 2003 user attempts to open it, they receive
a message that they should not open it unless received by a trusted source,
click OK. Then, they receive warnings that that the file may be damaged. It
takes a long time to open. When it finally does open, there is no
formatting. Also there is a message that there was so much damage to the
file that Excel could not repair it.
At least two of the XP/Office 2003 machines have the 2007 compatibility
pack installed.

Any ideas what happened and how to fix? Appreciate your help and
assistance.