Can't get add in to load/debug

[softwareguy74]

I'm attempting to do something that should be rather trivial, but I'm having no luck at all. Here is the setup:

Windows XP 64
Visual Studio 2008 SP1
.NET Framework 3.5
Offfice 2003

Created a new Office/2003/Excel 2003 Add-in project. Put the following code in ThisAdIn.vb:

Public Class ThisAddIn
    Private Sub ThisAddIn_Startup(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Startup
        ' Start of VSTO generated code
        Me.Application = CType(Microsoft.Office.Tools.Excel.ExcelLocale1033Proxy.Wrap(GetType(Excel.Application), Me.Application), Excel.Application)
        ' End of VSTO generated code
        MessageBox.Show("ThisAddIn_Startup")
    End Sub

    Private Sub ThisAddIn_Shutdown(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Shutdown
        MessageBox.Show("ThisAddIn_Shutdown")
    End Sub
End Class

Try to run/debug it, and get the following message:

Unable to start debugging. Check for one of the following:

1. The application you are trying to debug uses a version of the Microsoft .NET Framework that is not supported by the debugger.
2. The debugger has made an incorrect assumption about the Microsoft .NET Framework version your application is going to use.
3. The Microsoft .NET Framework version specified by you for debugging is incorrect.

Well, I have set the target framework to 2.0, 3.0, and 3.5, and each one gives me the above error.

Does anyone know what may be causing this? This machine is a brand new (clean) machine with very little else installed. I just don't understand why this seemingly trivial task is being so darn stubborn!