Update label from another thread

I use a thread writing in another class for update a label. The label is contents in Winform Main class.

 Scanner scanner = new Scanner(ref lblCont);
 scanner.ListaFile = this.listFiles;
 Thread trd = new Thread(new ThreadStart(scanner.automaticScanner));
 trd.IsBackground = true;
 trd.Start();
 while (!trd.IsAlive) ;
 trd.Join();

How you can see, i pass the reference of label into constructor of the second class. In the second class(Scanner) i've a method called "automaticScanner" that should update the label with this code:

public Scanner(ref ToolStripStatusLabel _lblContatore)
{
        lblCounter= _lblContatore;
}
Thread threadUpdateCounter = new Thread(new ThreadStart(this.UpdateCounter));
threadUpdateCounter.IsBackground = true;
threadUpdateCounter.Start();
while (!threadUpdateCounter .IsAlive) ;
threadUpdateCounter.Join();

private void AggiornaContatore()
{
  this.lblCounter.Text = this.index.ToString();        
}

I've receive this error on update of label:

Cross-thread operation not valid: Control 'Main' accessed from a thread other than the thread it was created on

I use .net 4 with Winform C#.

Thanks a lot for answers.

News: The problem is this line:

trd.Join();

This line block my GUI and the lable was not update. There are methods to control the finish of thread and updating the label until the end? Thanks