You would need a separate partition for each OS, and every time you boot it would ask you to choose which one you want.
Alternatively you can to have several hard drives with one OS on each (on its own seperate partition). This way you can select which drive you want to boot from in the bios which removes the selection menu. Some people prefer to have the selection menu, others like the bios selection so windows boots up without needing any input.
You need at least 60 Gb for an OS partition these days, I have mine at 80 Gb and with Vista it only had about 15 Gb left, with Windows 7 there is 30 Gb left.