No you don't have to format your machine or loose your windows vista. Ubuntu can he installed on a different partition by partitioning your hard disk into two or more partitions.
Also ubuntu will create a GRUB boot loader, so that you can boot to your choice of operating system : Ubuntu Linux or your Windows vista.
Here are answers which will help you
How to install ubuntu on a windows xp system
How to install ubuntu on a windows machine

Ubuntu 11.04 code named natty narwhal is released download ubuntu 11.04
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