I have successfully tested this on Windows XP and Windows 7:
const
MONITOR_ON = -1;
MONITOR_OFF = 2;
MONITOR_STANDBY = 1;
To turn off the monitor:
SendMessage(Application.Handle, WM_SYSCOMMAND, SC_MONITORPOWER, MONITOR_OFF);
To turn on the monitor:
SendMessage(Application.Handle, WM_SYSCOMMAND, SC_MONITORPOWER, MONITOR_ON);