I've installed Ubuntu 14.04 and whenever I get here,
it says that root file system is not defined.
Try Ubuntu before you install option on boot menu and I booted it with UNetbootin.
How can I fix this?
The root filesystem is defined as
/ (slash), so you must select the mount point for some partiton as
Note that the partition must have a defined format type. Ext4 is a good one you can choose.
If there is no data inside, I use to prefer to
Format the partition.
If you want to KEEP your Windows installation, to allow a dual-boot system, then you will first need to back up your hard drive so when things go wrong you can start over without losing data.
After you ignore that advice, you will need to resize your Windows partition to free up some space for a new Linux partition. Your Windows partition would be the biggest ntfs partition in your image. You should probably make a note of how much free space you have when you're booted into Windows before you do this; make sure to leave some of that free space for Windows to use.
After you resize your Windows partition, you should have some unallocated space. Ubuntu's installer should be able to use that space quite happily.
The Ubuntu live CD or USB image you're currently running from should have some utilities for working with partitions; for example, "gparted". Good luck!
Double click on the disk partition in which you wish to install the OS, change the disk partition to ext4 and it will work fine.