I have shared the following text demonstration to install windows vista with existing xp.
Dual or multi-boot lets you have access to more than one operating system on your computer. A dual boot system lets you experiment with a new operating system or run software that’s not supported by your regular operating system. You can configure your computer to dual boot Windows XP and Windows Vista by installing the two operating systems on different partitions or hard drives in your computer, then selecting which operating system you want to load from a boot menu when you start your computer.
> Create two partitions on your existing hard drive or ensure that you have two hard drives installed in your computer. You must install each version of Windows on a separate partition or hard drive.
> Configure your computer to boot from the CD/DVD-ROM drive in the BIOS.
> Insert your Windows XP installation disc in your CD/DVD-ROM drive and start your computer.
> Follow the on-screen prompts to install the operating system.
> Run Windows Update when Windows XP is installed to update the operating system with the latest fixes and enhancements.
> Insert the Windows Vista installation disc in your CD/DVD-ROM drive. Reboot your computer.
> Install Windows Vista. Enter the activation key and accept the license terms.
> Choose “Custom” on the “Which Type of Installation Do You Want” screen.
> Select the second partition or hard drive. Click “Next.”
> Follow the on-screen instructions to install Windows Vista. When the operating system is installed, run Windows Update to ensure that Vista is up-to-date with all fixes and enhancements.
> Remove the Windows Vista installation disk.
> Reboot the computer. When the computer restarts, you have a boot menu that shows Windows Vista and “Microsoft Windows Earlier Operating System.” Use the arrow keys on your computer to select the operating system you want to use and press “Enter.”
How to Setup a Dual Boot in Windows Vista
Microsoft Windows Vista can install in a dual-boot situation when you want to have more than one operating system available simultaneously on a computer. You can use your Microsoft Windows Vista installation disk to set up the dual boot for your computer. At the end of the process, you will be able to run both Windows Vista and another operating system on the machine.
> Power up your computer. Your current version of Windows will start in the normal manner. Put your Windows Vista installation disk into the drive.
> Select “Install Now.” You will be asked to get the latest updates for the computer if you have an Internet connection. Microsoft recommends this to boost your computer’s security.
> Enter your 25-character product activation key. Read all license terms and agree to them.
> Select the “Custom” installation type. The program will ask you where you want to install Vista. Choose the hard drive or partition to which you wish to install Vista.
Click “Next” to initiate the installation. When the installation completes, reboot the computer to gain access to Vista.
Sometimes users need duel boot operating system to get the hardware compatibility families. Following videos will help you to do that very easily. Here you also can get text demonstration just below the videos.