It is not clear what timer you are using; I see you at least tried to use System.Timers.Timer. As Cor indicated, there are actually three timers as in Form.Timer vs Timer.Timer vs Thread.Timer.
Timer tick event handlers execute in another thread. I think you know that since you say "multithreading". You cannot do anything with the window from that other thread. You must do something that switches to the main thread. That is why you should use the fimer in the System.Windows.Forms namespace. Its tick handler is automatically in the UI thread. Also see Timer Component. At the bottom is a link to Timer Control for Visual Basic 6.0 Users.
Sam Hobbs
SimpleSamples.Info
