1. It is important to back up all your files before starting the upgrade. You will need your previous Windows XP CD and a MS-DOS boot CD. It is relatively easy to create a MS-DOS boot CD: visit the allbootdisks website and download the files and follow the instructions.
2. You will need to ensure that your MS-DOS boot CD loads up before Vista. To do this, make certain that your CD-ROM drive is given priority over other drives. You can do this by going to your BIOS settings and selecting this option if it is not already checked.
3. Insert the MS-DOS boot CD in your drive and then restart it. A popup setup menu should appear. When it does, select or type "R" to repair a present installation in the setup wizard.
4. Following the above action, type the next command:
fixboot C:\
5. Next type the following command:
fixmbr C:\
Then type “exit.” The setup is complete and you can now restart your PC. Your MS-DOS boot CD should remain inserted into its CD-ROM drive as the computer is restarted.
6. Finally type the command:
format C:
as the command prompt appears once your computer is restarted. Your hard drive will then be formatted. Keep in mind that this will erase all data from your hard drive. Now restart your computer with the Windows XP installation CD inserted in your CD ROM drive.
7. The Windows XP set up menu will appear. Now just simply follow the wizard to install Windows XP onto your PC.