In case you have problem getting the Primary Interop Assembly (PIA) working fine with your .NET Visual Studio platform, just check this link.
Installing the PIA for Office 2003
For example I kept having a "tlbimp.exe" generated assembly (not the PIA) when adding a COM Reference to Microsoft.Office.Interop.Excel, from the "Add Reference", COM tab window.
This means the the .dll path of this reference was pointing to a local dll (local to the project), rather than the one registered in the GAC.
I sorted the problem just changing the "Copy Local" property to false, removing the reference and adding it again.
However, you could also have had some problem with the registration of the PIA in the GAC (Global Assembly Cache). The above link help you sort this kind of problems.