How to install Ubuntu 12.10 on already i.e preinstalled win 7 or win8 PC?

How to install Ubuntu 12.10 on already i.e preinstalled win 7 or win8 PC?

How to install Ubuntu 12.10 on already i.e preinstalled win 7 or win8 PC?
Please tell me with out deletion of my files..nothing should happen to my PC files which i installed on WIN 7 or 8