Using MS Office Excel 2003 on XP

I have written a number of macros that use the following code,

With Application.FileDialog(msoFileDialogFilePicker)....

Something (?) has happened and now this portion doesn't work any longer.  When clicking on the macro I get the error variable not defined with the msoFileDialogFilePicker highlighted. 

When I look at the references I don't have the choice of the Microsoft Office 11 Object Library.  The macros have been working for several months so I know that the code is OK. 

What dll could have changed or is missing?

Thanks

Susan



Software/Hardware used:
Excel 2003 on WinXP