Not possible xp installation (by 2 users)

How to Downgrade Vista to XP Downgrading from Windows Vista to Windows XP is almost like installing Windows XP for the first time. You should take all the normal precautions like backing up all your important files and record important passwords in a different place, preferably on a CD. When you downgrade from Windows Vista to XP, note that Windows Vista will be completely removed from your system.
(Note: There is a way to have both Vista and XP in a dual boot configuration, but that's an option for more advanced users. See Dual Booting Vista and XP for more info on that.)
Before you begin the XP installation, make sure that the necessary XP hardware drivers are available, or included on the XP disk provided by your PC vendor. If in doubt, check the vendor's website or poke around with your favorite search engine to see if others have attempted the XP install on the same hardware. You may be able to install XP without all the needed drivers, but some hardware may not work afterwards.
Also, make sure that you have a genuine Windows XP CD before you begin the downgrade. Be sure to read the section earlier in this article to see if you qualify for a low/no cost XP recovery disk from your PC vendor. If not, perhaps you have an XP disc that came with another computer. If you've upgraded an existing computer from XP to Vista, the best choice would be the XP install/recovery disc that came with that computer. One some PC's the vendor doesn't ship a CD, but the recovery CD can be created from disk images on the hard drive. If all else fails, you can always buy a copy of Windows XP on Ebay. Just be sure it's a legal, licensed copy.
Follow these steps to install Windows XP on your Vista machine:

• Insert your Windows XP install/recovery CD and restart your computer.
• Install Windows XP normally, providing either the product key that came with the disk, or the one you got from Microsoft customer support. If the installer warns you that another copy of Windows is already installed, don't worry. Remember, this install will wipe out your existing Vista installation.
• Activate your copy of Windows XP when it prompts you to do so. If you run into the "already activated" problem, you can use the option to activate by phone.
• If you own Vista Business or Vista Ultimate, explain to the rep that you want to use your Downgrade Rights to install XP Pro.
• If you own Vista Home Basic/Premium, or you are installing XP Home Edition, don't mention ANYTHING about Vista. Just give the rep your XP product key (not the Vista key), explain that you are installing XP on a new hard drive, and tell them you need to activate.