First of all you can create Partition your hard drive. In Windows XP, using the $50 Partition Magic the free GParted Live CD, create a new primary partition to install Vista on. Make it at least 20 gigabytes in size and then Install Vista on the new partition. Pop in the Vista CD and point it to your newly-created partition as the installation destination. Installation will restart your machine at various points.
and then Choose your operating system on boot. Once Vista is installed on your newly-created partition, on boot up, you'll get a choice to start XP or Vista. The default is Vista. However, this can be changed using Vista's boot manager.