Problem sharing spreadsheet between Office XP and Office 2003

I have an Excel spreadsheet that I created in Office 2003, and it
includes the SOLVER add-in. When I open the same spreadsheet on a
different PC on our network, which has Office XP (and SOLVER is
installed), the Solver add-in doesn't work. When I go to Tools -
References on the Office XP PC, it says "MISSING: SOLVER.XLA". It is
looking for "C:\Program Files\Microsoft
Office\OFFICE11\Library\SOLVER32.EXE", which of course doesn't exist
on the Office XP PC, because Office XP has "C:\Program Files\Microsoft
Office\OFFICE10\", not "C:\Program Files\Microsoft Office\OFFICE11\"
(Office XP = Office10 and Office 2003 = Office11).

Before I go any further, I just want to say how counterintuitive I
think it is that Office add-ins are set up this way. By that I mean
how it stores the add-ins in folders named "C:\Program Files\Microsoft
Office\Office9\..." (Office 2K) or "C:\Program Files\Microsoft
Office\Office10..." (Office XP) OR "C:\Program Files\Microsoft
Office\Office11..." (Office 2K3), etc. What do they want you to do
when you're sharing Office files that have references to add-ins in
such folders, yet, since you're using a different version of Office
than that of the person who created it, that folder doesn't exist?
Wouldn't it make much more sense to put these add-ins in a neutral
folder that all versions of Office will have?

Now once I open that spreadsheet in Office XP, I can simply uncheck
the MISSING: reference and check the Office10 version of SOLVER, and
it will work fine. Then of course I have to change the reference back
to Office11 when I open it on the Office 2003 PC. It's a pain to have
to do that every time I and my colleagues open the file.

I tried placing the SOLVER add-in in a neutral location (C:\SOLVER)
and deleting it from "C:\Program Files\Microsoft
Office\Office11\Library\Solver" and then installing it. Guess what?
When I installed the add-in from C:\SOLVER, it copied the files back
to "C:\Program Files\Microsoft Office\Office11\Library\Solver" and ran
the add-in from there again. It was a good try though.

I would like to know how I am supposed to resolve this whole issue
(especially what Microsoft wants you to do in this situation). Of
course, we could upgrade all of our PCs to Office 2003, but we'd like
to avoid that costly solution at this point. Besides, Office12
willcome into play at some point when we acquire new PCs in the
future, and then once again our network will not have a uniform
version of Office. Someone told me that there is a way to resolve
this issue by inserting Auto_Open code in the spreadsheets that
contain such add-ins, but I don't know what that code is. We also
have dozens of spreadsheets like this, so the code would have to be
added to all of them. Still, if that is the only solution, I'll do
it. Apparently there is VB code that looks for missing references and
refreshes using the appropriate add-ins that are installed on your
computer, but I don't know what that code is.

I would greatly appreciate any help on this issue. I'd imagine that
I'm not the only one to have this issue, seeing as how people share
Office files (containing references to add-ins) among different
versions of Office all the time.