I know this is simple but the first thing that you should do as you have just bought it and it should still be under warranty is to take it back where you bought it and explain your problem. This way they will not tell you that you have changed something in the meantime and not validate the warraty close.

To answer your question ,i am on that very notebook and i assure you it should have come with vista premium on the 5920.mine has the vista sticker on it,does yours?well im sure it will be easy to get taken care of not hard to switch it and if under warrant y just tell them the problem,hope that helps.