delete temporary files from word

I'm trying to change the properties of a word programmatically. 
To do this I created a windows service from. Net.

I have this code 

 Sub ModificarPropiedadesWord()
        Dim filePaths As String()
        filePaths = Directory.GetFiles("C:\Words\")
        If filePaths.Length > 0 Then
            For Each fi As String In filePaths
                If Wapp Is Nothing Then
                    Wapp = New Word.Application
                End If
                If docWord Is Nothing Then
                    docWord = New Word.Document
                Else
                End If
                Dim info As New FileInfo(fi)
                Try
                    docWord = Wapp.Documents.Open("C:\Words\" + Path.GetFileNameWithoutExtension(info.Name) + ".doc")
                    Dim _BuiltInProperties As Object = docWord.BuiltInDocumentProperties
                    If Not _BuiltInProperties Is Nothing Then
                        _BuiltInProperties("Author").Value = "TEST"
                        _BuiltInProperties("Subject").Value = "TEST"
                        _BuiltInProperties("Manager").Value = "TEST"
                        _BuiltInProperties("Company").Value = "TEST"
                        _BuiltInProperties("Category").Value = "TEST"
                        _BuiltInProperties("Keywords").Value = "TEST"
                        _BuiltInProperties("Comments").Value = ""TEST"
                        docWord.Saved = False
                        docWord.Save()
                    End If
                    If Not docWord Is Nothing Then
                        docWord.Close()
                        docWord = Nothing
                    End If
                    If Not Wapp Is Nothing Then
                        Wapp.Quit()
                        Wapp = Nothing
                    End If
                Catch ex As Exception
                    Dim w As New IO.StreamWriter("C:\logDAL.txt", True)
                    w.WriteLine("ExcepTion modify properties" & ex.ToString() & "  " & DateAndTime.Now.ToString())
                    w.Close()
                End Try
            Next
        End If
    End Sub

works well while the service runs continuously, but errors ifinterrupted.

I get the classic error 
"The process can not access the file is being because is used by another process"

because in the process keeps running background also generates temporary files and others like show in the image

sow mi question is , how i can handled this bugs=???