Problem with and Excel Add-in installing on customer's machi

[jwhite100]

I have a commercial add-in that is not installing properly on "some"
of my customer's computers. The add-in is written in Excel 2000. The
most recent support request came from a customer who has Excel 2002
with XP Home Edition. He originally tried to install it with
installation package I had created with Inno setup. When the
installation was complete a Installation workbook is automatically
opened with following code:

Option Explicit

Private Sub Workbook_Open()
Dim Myaddin As AddIn
Dim MenuBar As CommandBar
Dim muControl As CommandBarControl

For Each MenuBar In Application.CommandBars
  If MenuBar.Name = "Worksheet Menu Bar" Then
       For Each muControl In MenuBar.Controls
          If muControl.Caption = "SomeAddin" Then
             muControl.Delete
          End If
       Next muControl
   End If
Next MenuBar

For Each Myaddin In AddIns

  Select Case LCase(Myaddin.Name)
    Case "SomeAddin.xla"
      Myaddin.Installed = False
  End Select

Next Myaddin

Set Myaddin = AddIns.Add(Filename:=ThisWorkbook.Path &
"\SomeAddin.xla", _
        CopyFile:=True)
Myaddin.Installed = True

If Assistant.On Then
   With Assistant.NewBalloon
     .Icon = msoIconAlertInfo
     .Text = "The 45 day evaluation version of Someaddin is now
installed on your computer. To purchase this product select Purchase
from the Data Manager menu. To unlock SomeAddin with the registration
code sent to you after your purchase select Register from theSomeAddin
menu.  To create a new database go to the SomeAddin menu and select
SomeMenuItem then New."
     .Heading = "Installation Complete"
     .Button = msoButtonSetOK
     .Show
   End With
Else
   MsgBox "The 45 day evaluation version of Myaddin is now installed
on your computer." & Chr(13) & "To purchase this product select
Purchase from the Data Manager menu." & Chr(13) & "To unlock SomeAddin
with the registration code sent to you after your purchase" & Chr(13)
& "select Register from the SomeAddin menu.  To create a new database
go to" & Chr(13) & "the SomeAddin menu and select SomeMenuItem then
New.", vbInformation, "Installation Complete"
End If
End Sub

After this finishes running the clients say that the add-in does not
appear in the Add-in list and that when they open the folder
containing the add-in, the add-in is missing. For the last client I
developed a zipped version of the installation package and had him
manually install the add-in in Excel. When he tried this time he got
the message invalid Add-in and when he re-booted the file was missing.
Although he may have unzipped it to a temporary folder. I am not sure.

Although I am not sure how many times this has occurred, I have had
several report the same thing occurring but I didn't send them a
zipped version. This has occured with people using different versions
of Excel (2002 - 2003) and I can't recall if any have been using 2000
though. But I have had many customers who have had no problems
installing the add-in using the same versions of Excel and XP.

I was wondering if anyone had had a similar experience in the past or
knew what the cause could be.

Joseph