Actually, I'm kind of confused about this myself, because everyone says that you have to gain all 150 power stars to win, but you can actually win with only 80 stars, I've done it! All you do is get 80 stars, go to the top floor in the castle, go to the clock room, go up some stairs, and then there will be the endless staircase, but fortunatly, if you have over 80 stars, it ends. After you go up, go to the end of the room and jump in the little hole thingy, go threw the course, and at the end you will battle Bowser. After he is defeated, collect the shining star he elaves behind. You will start to fly, and do all these things. You will automaticly fly to the front of the castle, and Princess Toadstool aka Peach, will come out and thank you, and that's how to win the game. I hope that helped! enjoy!