Go to Start > Run, type "regedit" (without the quotes), click OK. Navigate using the little + signs to:
Click on that "Advanced" folder in the left pane, it's contents will show in the right-hand pane , right click on a blank part in that right-hand pane and create a new DWORD value, name it EnableBalloonTips (exactly as shown, with no spaces), and set the value to 0. If there is already an EnableBalloonTips entry, double click it and set it to 0. Exit the registry.
Make sure to backup the registry before editing it, though.