windows on top of another windows?

Well, to dual boot, you have to install a new windows on a different partition. For example:

When you install Windows for the first time, it will ask for you to make some partitions. You just need to make two (or more if you want) that give you enough space to install windows and save all your programs, etc. The, you can go ahead and install windows for the first time on one of those partitions.

Once you are done, pop back in the Windows disk, pick the other partition and install. And voila, you have two installs of Windows.