This question already has an answer here:
I would like to run Ubuntu alongside my current OS. It is Windows Vista. Do I just download Ubuntu or do I have to do something special? Thanks.
marked as duplicate by Luis Alvarado♦ Mar 14 '13 at 19:22
This question was marked as an exact duplicate of an existing question.
Having two operating systems on a hard disk is called "dual boot". It is much easier to install Windows first and Ubuntu afterwards, because Windows tends to overwrite the complete bootloader.
You don't need to do something special, just download and install Ubuntu. You will be asked if you want to run Ubuntu along side of Windows. The procedure is also covered in the community documentation.