This tutorial will tell you how to install ubuntu from USB flash drive,it may help if your computer don’t have a CD-Rom.

Also,you can create a bootable ubuntu live usb with UNetbootin.This is highly recommended if you prefer doing this with a graphical interface and this make things very  easy

1.First,download the ubuntu CD image file from ubuntu.com,and download “vmlinuz” and “initrd.gz” from here (for amd64) and here (for i386).

2.Then,in your root directory of your USB device,create a file folder named “install” and copy “vmlinuz” ,“initrd.gz” into this folder.

Double click the CD image file,and right click on “isolinux” folder,then extract to the root directory of your USB device.

Rename “isolinux” to “syslinux”,then go inside the folder and rename “isolinux.cfg” into “syslinux.cfg”.

3.Now,make the drive bootable.

For ubuntu user,run following in terminal:

sudo apt-get install syslinux

sudo syslinux /dev/sdb1 (use "sudo fdisk -l"to confirm "/dev/sdb1")

For windows user,download syslinux.zip from here,then extract it.

go to the file with command prompt,and run

cd win32

syslinux h: (change h: to your USB disk.)

4.Copy the CD image file to the root directory of your USB device.Now four files,or folders in your USB device:“syslinux”,”install”,”ldlinux.sys”,”.iso image file”.

Now,reboot computer from USB flash drive and start install ubuntu.

Tips:

You need a amd64 installer if you want to install an amd64 Ubuntu .iso image and the i386 installer for an i386 iso.