One thing I got to tell you when I am using Ubuntu, you will fall in love with it soon. You will feel it is yours.
As a prerequisite, I emptied one of my partition (not logical drive) other than the partition of Windows. All the files and folders were moved safely and made a backup. I decided to boot from USB for installing ubuntu. With the help of Power Iso, I made the USB drive a bootable disk. The destination is an ASUS X53S, Intel core i5,with 6GB of ram. But when I was trying to set the boot priority in bios to the USB, it was not seen there. But I discovered that, at the time of booting when the ASUS logo is displayed, keep pressing the ESC (escape) key. It will then ask to choose the boot location, including the USB drive. Upon choosing to boot from USB, it will start installing. Just follow the on screen instructions.
At one stage it will prompt to set the disk space for ubuntu. A movable central bar will be there to adjust the disk space allocation. In the left it is for ubuntu and in the right it will be retained by Windows. The total space we see in there is the emptied partition. You are done. The installation will proceed. Most probably next time when you boot, you will be prompted to choose the OS. If it didn't you need to do a small job. Boot from the USB once again. Open the terminal and type in the following commands. If there was no error while installing then you can re-install your GRUB as follows:
First, boot from a live CD
Second, open your terminal and write sudo fdisk -l to see all your partitions and exact name of your linux one (sdxy) where x means the hard number (a,b,c,...) and y the number of partition (1,2,3,4,...)
Third, after that you should write these following instructions:
sudo mkdir /media/root
sudo mount /dev/sdxy /media/root
sudo grub-install --root-directory=/media/root /dev/sdx (without y number!)
You are done. Next time you will be prompted to choose the OS. That's all. Enjoy
Foot note : To make the USB back to a normal USB, using the Power Iso, write in some normal iso files (in the same way you created the bootable USB). Now you can format like the normal procedure. Because sometimes a normal formating process will not take it back to the normal functioning.