If you are running Ubuntu and Windows on the same machine with help of a boot loader, you might run into to problem that the wrong operating systems starts when you start your computer.
There are two ways to fix this, the hard way and the easy way, I will show you the easy way.
Boot your computer into Ubuntu, go to the ”Software download center”, search for ”grub”, the first option that pops up is ”Start-manager”, install that application. When it is installad just go to ”Settings -> administration -> Start-manager” then you will find a dropdown with the label ”Default operating system”. Just select your operating system, close the window and reboot your computer.