Check the accessory (serpentine) belt tensioner for correct operation. They sometimes seize or lose the ability to apply proper spring tension to the belt. Your accessory belt likely needs replacement if it's old or has been slipping for a while.
Belt squeal on start up is usually the belt slipping on the idler/drive pulleys. It can also be the idler pulley bearing failing. Curing the problem simply involves replacing the affected parts.