First all you need is the Ubuntu ISO file which is available for download on the official site. You can download the latest updated version on the link below :
Select the Ubuntu variant according to your need. For normal users, Ubuntu Desktop is Recommended.
You can choose between 32 bit and 64 bit version of the OS. If your system or PC is compatible with 64 bit architecture go for it as it has capability to address more memory (more than 4 GB) than the 32 bit version (capable of addressing less than 4 GB ram). 64 bit OS on compatible system are more stable and have good performance over 32 bit system.
After you download the ISO file, Open it using any ISO image utility (PowerISO, UltraISO, MagicISO etc). I use PowerISO ( Torrent Download Link ) to extract the file name wubi.exe to same directory where you have saved your Ubuntu ISO image file. Simply drag that file to your directory. I suggest you to put both ISO and wubi.exe file in your root directory before installation. i.e in your D: or E: not inside any folder.
After extracting that file (wubi.exe) to the same directory of Ubuntu ISO file. Open wubi.exe and it prompts these dialog box. Select Install inside windows to install Ubuntu alongside with windows.
Adjust your appropriate configuration on this dialog box that appears right after the above one. You can install on C: or the drive where your current OS may be windows 7 is installed. Note that the user name and password on this Ubuntu Setup is the username and password used to log in your Ubuntu OS.
After that it asks for reboot and after reboot it starts with Ubuntu Installation. Follow the instruction on the screen. You will be prompt during system startup which OS you want to boot from the disk. select Ubuntu to boot form the hard disk.
After the first time installation is complete you can boot Ubuntu by selecting it from boot screen above.
All done… That’s Easy…!