When you install just about any linux distro (including Ubuntu) it will partition the drive for you and install a boot loader (such as Grub). This presents you with a menu before the OS boots asking you to choose - usually has a timeout to boot the default OS if you don't select one.
Make a good backup first!
These days installing Linux and dual booting is pretty reliable but its well worth making a good backup (preferably a hard drive image) before hand - just in case
Thanks, I have a regularly updated file on our NAS so I'll be pretty safe.
I almost installed it the other day but as the install wizard was loading dad called because his Citroen CX broke down and I had to ferry some tools to him and it sort of just slipped my mind.