The install program for Ubuntu will give you the option to use as much of your hard drive for Ubuntu as you wish too without affecting Win 7 or XP at all. I have installed many different Linux's along side windows without any problems at all in a dual boot system.
I prefer to use a USB flash drive or a SD card though as they are easier to get rid of or change to another Linux at a later time than redoing a dual boot system. There is a program called LinuxLive that will install Ubuntu to a flash drive or SD card and put a file on your drive to save data and add on programs at a later time after installation.
I have Mandriva One; Ubuntu and JOli 1.2 installed to three such drives and they all work just fine. fdalbor