Excel 2007 imcorporated some functions which used to be in a add in previous
versions. This is the case, for example, for the workday() and networkdays()
functions but there are some others.

When I open a 2003 spreadsheet with this functions Excel prompts I'm using
"reserved" names for user defined function which is now internal.

The problem is when I save this 2003 spreadsheet in Excel 2007 (in a .xls
format) and try to use it in Excel 2003 again, because something happens and
I completely loose this formulas (sometimes I start have the #VALUE! error or
even worse problems).

Does anyone had similar problems and now how to solve it?