If you delete the partitions, should be ready for Windows.
But let's be clear here. Microsoft has never supported installing Windows and leaving other OSes standing. So you would delete all the partitions leaving an empty drive. Now I can boot the Windows DVD and install.
But let me comment about the original post. I never had to shrink partitions like you noted with Ubuntu's installer. I would boot my Ubuntu DVD and it's installer would ask if I wanted to use the entire drive or if it should make space for Ubuntu.
At no point was I shrinking partitions. I think you overcomplicated the Ubuntu install.