problema con i thread

[albertozzo]

Salve,

Ho un problema con i thread:

ho eseguito questo piccolo esempio:

    Private Sub bottone1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles bottoneConnetti.Click
        Try
            'Declare a Thread and Assign it to a Methord Hi 
            Dim t As New Thread(AddressOf tr1)
            'Declare Another Thread and assign it to Bye
            Dim b As New Thread(AddressOf tr2)

            'Set the priority level for each thread
            t.Priority = ThreadPriority.Normal
            b.Priority = ThreadPriority.Lowest

            'Start the thread execution
            t.Start()
            b.Start()
        Catch ex As Exception
            MsgBox(ex.ToString)
        End Try
    End Sub

    Public Sub tr1()

        Do While True
            txtBox1.Text += "prova1"
        Loop


    End Sub

    Public Sub tr2()
        Do While True
            txtBox2.Text += "prova2"
        Loop
    End Sub

Mi restituisce come errore "Cross-thread operation not valid: Control 'txtBox1 accessed from a thread other than the thread it was created on."

Come mai?
Come posso risolvere?