The normal Ubuntu download that is not Wubi installs in a new partition. Like just about every other Linux distribution, such as Red Hat, Debian, etc, it will look for any other operating system, like Windows, and create a dual boot configuration for all of them.
But be aware that once you install any of them, you can't simply erase it because the boot configuration file is on the last one installed. If you delete it, Windows will not boot unless you restore its boot loader using either the original Windows boot disk, or a 3rd-party utility.