BackgroundWorker.IsBusy Property

private void downloadButton_Click(object sender, EventArgs e)
{
    // Start the download operation in the background. 
    this.backgroundWorker1.RunWorkerAsync();

    // Disable the button for the duration of the download. 
    this.downloadButton.Enabled = false;

    // Once you have started the background thread you  
    // can exit the handler and the application will  
    // wait until the RunWorkerCompleted event is raised. 

    // Or if you want to do something else in the main thread, 
    // such as update a progress bar, you can do so in a loop  
    // while checking IsBusy to see if the background task is 
    // still running. 

    while (this.backgroundWorker1.IsBusy)
    {
        progressBar1.Increment(1);
        // Keep UI messages moving, so the form remains  
        // responsive during the asynchronous operation.
        Application.DoEvents();
    }
}