that reminds me - I once read the Napoleonic wars being referred to as the First World War, and the First and Second World Wars are sometimes considered as a single war with a period of readjustment in between. So we have had one, two or three world wars, depending on the definition.
Referring to conflicts as 'The Hundred Years War' or 'The Second Hundred Years War' is just for the sake of simplicity. In reality more than one conflict takes place in a hundred years, almost by definition.
Thing is bernardo that the Napoleonic wars only invloved European coutries as far as I know and there for they couldnt really be refered to as a World war. This is obviously not the case for the first and second where the war involved people from every continent