My name is Willem and my ancestors come from the Netherlands. My father and my grandfather and myself are all named Willem Petrus. The pronunciation is using the W as a V. So it could be spelt as Villem. Hope it helps...
Just to clear up the V/W issue; Willem is pronounced with a Dutch w-sound, which is different from an English w and not quite a v either. You can listen to it here: http://www.forvo.com/word/Willem/#nl.