There are a couple of ways of doing this. If you only need the worksheets and none of the code, then the simplest method is probably just to copy out the worksheets to a new workbook then save that:
Private Sub workbook_open()
Dim wb As Workbook
Dim saveName As String
ThisWorkbook.Sheets.Copy
Set wb = ActiveWorkbook
saveName = Application.GetSaveAsFilename(fileFilter:="Excel Workbook (*.xls), *.xls")
If Not saveName = "False" Then
wb.SaveAs saveName
End If
End Sub
If some code is still required in the XLS then you would need to manipulate the VB environment directly to remove the parts you didn't need. This is a bit more involved and has some important restrictions/caveats, but it sounds like the simpler method illustrated above may suit your needs anyway.