Whenever you upgrade to Vista by Windows XP, you may be detecting it difficult to revert back up to XP. In that respect are a lot of factors involved in uninstalling Vista and a few by this arrive at it hard to successfully go back to XP. If you answer a new Operating System upgrade, you demand to aspect the information that relapse back to a previous version may not be an option and even if it were an option, the process of going backward is complicated and may not even work.
Therewith supposed, be sure you have made the necessary backups before upgrading to Vista or before attempting to downgrade your Windows Vista installation to XP. You will be able to try to downgrade without turning a loss data point* via the Microsoft Knowledge Base article at http://support.microsoft.com/default.aspx/kb/933168.
Merely the best way is to use a fresh XP CD on a fresh permit key. Although you are able to attempt to use a previous license of XP from that computer, it may not work, as there have been issues with Vista rendering previous XP licenses useless after a Vista upgrade. This makes sense, as Microsoft does not want you to sell the upgraded XP after you are no longer using it. So be sure to get your hands on a boxed copy of XP before attempting an upgrade.
Boot to the fresh copy of XP. To do this, you might ask to go into the arrangement BIOS to change the first boot device to the CD ROM drive. When asked where you want to install XP, look for the current partition where Vista is installed and remove it. These will wipe out the Vista installation and all data will be lost. You are able to now install XP by choosing the free space on the hard drive.
You’ll besides ask to establish device driver* as altogether of your hardware. Whenever you do not accept a device driver disk, make sure to go to the manufacturer of the hardware (motherboard manufacturer, video card manufacturer, sound card manufacturer, etc…) and have those disks ready.
Closure Summary:
1.    Backup all documents earlier you assay to uninstall or reinstall an Operating System.
2.    Best to use a fresh Windows XP license and an XP disk.
3.    Boot to XP disk, remove Vista parition and then install XP.