Just to clarify a few points, you have one drive (1TB) and you have windows 7 installed on it and it boots, correct?
This is an MBR install, not UEFI?
You are not using fastboot or hibernation?
You did create unallocated space on the drive from windows?
The 'root partition' error you report is pretty common when no mount point is selected for root (/) in the installer. Maybe if you posted the output of: sudo fdisk -l it would give us more information.
Have you tried the manual method, something else?
The link below has a detailed tutorial on installing 14.04 as an MBR install only.
http://www.dedoimedo.com/computers/u...all-guide.html