If you wish to boot Windows Vista and OS X without erasing the latter, use boot camp which is included in OS X 1.5 Leopard. In case if you have already installed Boot Camp on your Mac computer and wish to upgrade to Windows, you do not need to re-install Boot Camp. You must be logged in as an administrator in order to install Microsoft Windows Vista. Also, to set up Windows Vista on your Mac computer, you’ll need the following:

• An Intel-based Mac computer
• A built-in keyboard or a USB keyboard, a mouse or a trackpad
• Mac OS X version 10.5 or later (the latest version of Mac OS X is strongly recommended)
• 10 GB of free space on the disk that you are installing Windows on
• A licensed copy of Windows Vista Home Basic, Home Premium, Business, or Ultimate

To configure Windows Vista, now you can follow the steps below:  

• First, take a back up of all information stored in your computer
• Use the finder and browse through applications, utilities and run Boot Camp Assistant
• Click Continue on the Introduction screen that appears
• Next, you will need to specify how you want your hard drive divided up for Windows. When you are ready, click Partition and your disk will be prepared

• Now, you can insert your Windows Vista install DVD or CD and click on Start Installation. Your Mac will automatically reboot and boot to the install DVD
  
• Select the partition called BOOTCAMP to install Windows Vista. Once it is installed remove the Windows Vista DVD/CD
• Next, insert your OS X 10.5 Leopard install DVD and run the Boot Camp installer application to enable Windows drivers for your Apple hardware

Alternatively, you can also print and follow the instructions in the Boot Camp Installation & Setup Guide while installing Windows Vista.