I suspect your problem is that you are not selecting, and using, a separate partition for the Windows 10 installation.
Create another partition. Call it something you will recognise (Win10?)
Boot up your ISO DVD and work through to the second Window , "Install Now"
Click this and then agree with the eula in the next window.
The next window will give you the option to "customise". Select this and choose your new partition. click next and sit back for 15 minutes or so.