Assign a process to a word.application variable in VB.NET

I am automating a word document which is getting generated each time i click a button in a web page, i am able to get hold of that WINWORD.EXE process but unable to set it to a word type variable. This is what i had tried:

    Dim ProgID As String = "Word.Application"
    Dim app as Word.Application
    Dim doc as Word.Document
    Dim p() As Process = Process.GetProcessesByName("WINWORD")
    For Each itm As Process In p
        If InStr(itm.MainWindowTitle, LetterType) > 0 Or 
            itm.MainWindowTitle = "" Then
            Dim retVal As Object = Marshal.GetActiveObject(ProgID)
            Try
                app = Marshal.GetActiveObject(ProgID)
                doc = app.ActiveDocument
            Catch ex As Exception
                MsgBox(ex.Message)
        End Try
        End If
    Next

Dim reVal as Object = Marshal.GetActiveObject(ProgID) throws error:

System.Runtime.InteropServices.COMException: 'Operation unavailable (Exception from HRESULT: 0x800401E3 (MK_E_UNAVAILABLE))'