'The process cannot access the file ... ' error

Private Sub secondForm_FormClosed(ByVal sender As Object, ByVal e As System.Windows.Forms.FormClosedEventArgs) Handles Me.FormClosed    
ImageList1.Images.Clear()
                ImageList1.Dispose()
                ImageList1 = Nothing
                treelist1.Items.Clear()
                treelist1.Dispose()
                treelist1= Nothing
                bitmapViewer1.Bitmap.Unlock()   'Just in case it is locked, but we never use .Lock method
                bitmapViewer1.Bitmap.Dispose()
                bitmapViewer1.Bitmap = Nothing
                bitmapViewer1.Dispose()
                bitmapViewer1 = Nothing
                bitmapViewer1.Bitmap.Unlock()   
                IO.Directory.Delete("C:\fld\", True) 'Here comes the error 'process cannot access the file......
End Sub

For Each frame As TiffFrame In tiffReader
                Dim thumbnailBitmap As New Aurigma.GraphicsMill.Bitmap
                frame.GetThumbnail(thumbnailBitmap, thumbnailSizeWidth, thumbnailSizeHeight)
                Dim thumbnail As Aurigma.GraphicsMill.Bitmap = CreateThumbnail(thumbnailBitmap)
                thumbnailsView.Items.Add("Page " & (number + 1).ToString(), number)
                thumbnailsView.Items.Item(number).Tag = arrayTagData
                imageList.Images.Add(thumbnail.ToGdiplusBitmap())
                number += 1
                frame.Dispose()
                thumbnail.Dispose()
                Exit For ' I need only the 1st page in case of multi page tiff
Next