How to use an specific version of Excel in a C# program

Question

For many reasons, I have installed on my laptop MSExcel 2003 and MSExcel 2007. I need both versions to develop specific projects for each version (Document level projects and Application level projects). Now I need to do a WinForm project that opens an Excel file, read a CustomXMLParts and write a new Excel file. I'm using a reference to Microsoft.Office.Interop.Excel that use ..\Visual Studio Tools for Office\PIA\Office12\Microsoft.Office.Interop.Excel.dll library (it's for Excel 2007). And for this code:

Microsoft.Office.Interop.Excel.Application excelApplication;    
excelApplication = new Microsoft.Office.Interop.Excel.ApplicationClass();
string version = excelApplication.Version;

At this point version is "11.0", but I need to open Excel 2007 and it must be "12.0", and then when the program try to get the CustomXMLParts, throws an Exception because this method doesn't exist in 2003.

If I uninstall Excell 2003 it works fine, but I need to work with both (2003 and 2007). When I reinstall Excell 2003, it fails again. I check the property "Specific Version", for the Interop.Excel reference, to be sure that it's true, and I tried to modify the app.config oldVersion="12.0.0.0" to not make compatible with Excel 2003, but nothing happens:

  <assemblyIdentity name="Microsoft.Office.Interop.Excel"  publicKeyToken="71E9BCE111E9429C" culture="neutral"/>
  <bindingRedirect oldVersion="0.0.0.0-11.0.0.0" newVersion="12.0.0.0"/>

Any idea?

I'm using Visual Studio 2008 and Visual Studio Tools for Office.Thanks in advance.