1. Your first step will be to modify the Windows XP system partition to make space for Vista. (You Have to Install Vista into a Different Partition Used for Windows XP)
2. Restart the system and boot from the Vista DVD.
Work your way through the Vista install screens until you get to "Which type of Installation do you want?"
3. As this is not an upgrade install, select Custom install.
4. Select New Partition that not Used for XP
The Vista boot manager will take over the system completely, and Windows XP effectively loads via Vista. It’s all pretty seamless though, and you shouldn’t encounter any technical problems.
5. Once Vista is installed and the system reboots, you’ll be presented with a boot menu with two options: “Microsoft Windows Vista” and “An Earlier Version of Windows”.