If you wish to install Windows XP on a computer that comes with Windows Vista already installed then you will have to create a separate partition on your hard drive to install Windows XP onto. This effectively splits the hard drive into two, and keeps the different versions of Windows separate. To do this you need to open up the Computer Management screen by going to the Start menu and then right-clicking on Computer and selecting Manage.
From the Computer Management screen select Disk Management, right-click on your main hard disk where Windows Vista is installed and then select Shrink Volume. This opens up a window where you can choose how large you want the new drive partition for Windows XP to be. You need to select the size of the new partition carefully, as you will need enough space to be able to install Windows XP successfully (around 10 gigabytes) whilst leaving enough space for Windows Vista. After you have selected the appropriate amount of space click on the Shrink button to begin the process, and once that has completed you should be able to right-click on the new partition (which should be labelled “Unallocated free space”) and select New Simple Volume. Follow the on-screen instructions to configure the partition, set a drive letter (such as E:\), give it a meaningful name such as “XP”, and then restart your computer. Whilst your computer reboots, access the BIOS and change to boot sequence to boot from your CD/DVD Drive before any other device, insert your Windows XP installation disc and then save and exit from BIOS. You will then be taken to the Windows XP installation program where you should follow the instructions until you come to the screen asking you which partition to install Windows XP to. Make sure you select the new partition and not the one with Windows Vista installed, as this will cause problems. Once Windows XP has finished installing, you will notice that Windows XP loads up automatically, rather than giving you the choice of which operating system to use. To rectify this insert your Windows Vista installation disc and restart the computer to access the Windows Vista installation program. From the Install Now screen select Repair My Computer, and then select your Windows Vista installation and then Startup Repair. You will now be able to choose which operating system to use when your computer starts up.