If you install Ubuntu as a second Operating System alongside Windows, every time computer boots you will be presented with a Grub boot menu where you can make a choose which OS to boot. If you don't make any choice in 10 seconds your default OS will boot. Obviously after installing Ubuntu it will become the default Operating System.
You may want to change your default OS or reduce Grub timeout. Probably easiest way to achieve this is to install StartUp-Manager.

1. Open Terminal and run sudo apt-get install startupmanager and wait until installation is completed.
   
2. Open Dash Home, type "Startup" in the search box and open StartUp-Manager
   
3. In StartUp-Manager select your Default OS and reduce reduce timeout if required. Click Close to save changes