I have been using elementaryOS with dual boot for quite some time. I first installed eOS as dual boot next to my Windows installation in order to test things and not delete my Windows installation. But now I am so happy with eOS, that I want to upgrade my dual boot installation into a single boot (only elementary) installation. And is there a way to enable then full drive encryption?
The easiest way is to backup your data, wipe the disk, and reinstall elementary os. Full disk encryption is available as an option when you reinstall.