I have installed Window 8.1 on my HP laptop.Now I want to install Ubuntu 14.04,I try to use "Wubi" but when I use it, after installation on window, when my computer restart i have a message "No root file system is defined" I have read some thread here and i got some solution. One of them are "I must format my partition again, using ext4 format and mount on it". I did it by using Gparted in Try Ubuntu mode, but I got another message "/dev/sda contains GPT signatures,indicating that it has a GPT table. However, it does not have a valid fake msdos partition table,as it should. Perhaps it was corrupted--possibly by a program that does not understand GPT partition table. Or perhaps you deleted the GPT table, and are now using msdos partition table. Is this a GPT partition table?"