Technically its now referred to as a ribbed belt or serpentine belt. I am from the old school and will always call it a fan belt. even if the fan is in the boot.
Do not squirt the belt with WD40 it will stop it squealing because it wont work at all.The pulley will keep going round without taking the belt with it.However there is a product on the market that can be squirted on to it,I forget what it is called but it puts more grip on to the belt.
Thanks for all your answers - the fan belt was adjusted when he bought the car (August) but within a week it started screeching again. I recently took it to the garage for him to have front break discs and pads replaced - I mentioned it to the mechanic, he said it was getting stuck now and again and this was a fault on a Focus.
Tony - my old Dad said put a few drops of water on the belt to quieten it aswell.