Error on large Excel spreadsheet with VBA, written in '03 used inmixed '03 - '07 environment
 

Workbook is 28 sheets, most VBA code effects workbook formatting
(hidden sheets, password access, navigation) or protection and are
linked to buttons, toggles, checkboxes, DTpickers, combo boxes, etc.
A smaller version of this workbook was used last year without issue in
the same mixed Excel environment.

This year, after a couple of days of use, it will first cease
allowinng people to save data, if "save as" is used to save to a new
location Excel locks up, and then eventually start locking up, and
eventually refuse to open. At some point either before or at the same
time as the save issue begins, the VBA becomes unaccessible. If a
sheet/module's code is opened, an error message appears about a
missing DLL.

The Excel '07 users claim that they are always saving the workbook in
'03 format, and as stated earlier a substantially identical version
was used lasst year in the same environment without issue.

The workbook is a daily work tracking utility for a department of 22
with (user-defined date frame) reporting (that would probably be
better handled through a pivot table than the UDF/VBA/Functions method
I have employed, but you dance with the devil ya know...). It is 4MB
and can be emailed if someone wants to look that closely or is
curious.

Any ideas about what is happening, I am completely stumped...

thanks in advance