Why a thread? If you really need this to occur, set up a class level Timer, with an interval of 60 Seconds, and do the refresh in the Tick event.
Timer refreshTimer = new Timer();
...
refreshTimer.Interval = 60000;
refreshTimer.Tick += new EventHandler(refreshTimer_Tick);
refreshTimer.Start();
...
void refreshTimer_Tick(object sender, EventArgs e)
{
Invalidate();
}