I have a problem with Excel 2003 that manifests itself as follows: can't find project or library

Background
==========

Two years ago, I built a sophisticated payroll management system for contractors for a small business, using Microsoft Excel 2003,
extended by VBA.

It has been used successfully on a network of 3 PCs operating under Windows XP.

The system is based up on a workbook (call it XXX.xls), supplemented by a DLL written in VB6, named XXX.dll. Most of the VBA lies in the DLL to facilitate maintenance.

The workbook and its associated DLL are installed on each PC, with a shortcut to XXX.xls placed on the Desktop.

When activated, XXX.xls inquires a SQL Server database on a network server to display a drop-down combo-box of names and businesses.

Assuming the user picks a suitable candidate (say "MyBusiness"), XXX.xls will either:

create a workbook MyBusiness.xls in the folder "MyBusiness" on the server,
or open an existing workbook called MyBusiness.xls.

Problem
=======

The problem arose when a fourth PC was added to the network, and when a failed PC was replaced.

Double-clicking XXX.xls on either of these PCs produces the error message "can't find project or library", when the user selects any business,
but only on these machines.

The DLL is present and should be available. Control can be obtained by manipulating the Tools | References dialog, but this is very tedious, since there are more than 200 separate workbooks.

I have tried executing the following command when logged in as administrator:

regsvr32 C:\XXX\XXX.dll

but to no avail.

I have manually checked the list of DLLs used by Excel and my DLL on three of the four PCs (i.e. XXX.dll and other Microsoft DLLs),
looking for disparities. Though there are some, none should account for the behaviour observed.

Three of the four PCs share the same build state for Excel.

I am somewhat perplexed. I cannot help thinking that there might be some environmental settings (e.g. privileges, permissions, access rights) that are affecting the operation.

I have researched the problem, but not come up with a solution.

Has anyone encountered anything similar to this problem, and more importantly, found a solution?

I would be grateful for any insights or possible leads that might enable me to solve the problem.

Many thanks.