Step Wise Installation of Windows Vista in a Dual-Boot Setup

Installing Windows Vista without removing Windows XP | Deploying Windows Vista: A Power User's Toolkit

If you already have additional free hard drive or a separate partition, just follow the steps as per the clean installation process, but whole going for a dual-boot installation, boot with the Windows Vista DVD, choose Install, enter your product key and so on. You have to take care of the part, where setup reaches the screen that asks, 'Which type of installation do you want?', as shown in the following images.

Select the Custom (advanced) install type and click Next. Examine the next screen, which will be similar to the image displayed below. In it select the empty partition for sure, which later on getting assigned for Windows Vista and not for the older XP OS partition. You may rename it to any convenient name like Vista etc.

Setup will proceed, just like it goes ahead during the clean installation. When Vista gets installed, you may notice a difference, as soon as When you reboot the PC, a boot menu, the screenshot is displayed below, select earlier OS or Windows Vista.