The way I've done this is by creating two versions of an icon. One with a transparent background and one with a red background. Then, when I want the icon to "flash", I enable a Timer and in the timer_tick event, I change the MainForm's Icon to the normal icon, then back to the icon with the red background and back again, so forth and so on with each tick. Disable the Timer to stop the flashing (you might also want to set the icon back to the normal one, if desired). Adjust the Timer interval to make the flashing period to your liking. 500 ms is a good place to start. Don't make the interval too small or you'll induce seizure in your users.
