This is definitely possible, but you need to be careful before you try anything.
If the computer you are converting from Vista to XP is a new laptop, you will most likely NOT be able to use your wireless, sound, and other devices with Windows XP (I know, this happened to me). Even if your computer is a desktop, you will probably not be able to find all drivers. All computers (laptops and desktops) that are shipped with Windows Vista are new as of January 2007. Windows XP does not support all new drivers.
If you have both the installation disk for Windows XP and Vista, I would recommend that you set up your computer with what is called "dual-boot" where you can start your computer in either XP or Vista. This is the safest way to begin a conversion to Vista. The problem is, new computers with Vista installed do not come with the Vista DVD install disk. However, they do have a recovery “partition” which contains the entire operating system.
If you want to just risk it, and install XP (I hope professional, otherwise not much of an upgrade :S) over Vista (you will lose ALL your data, but not the recovery partition), it’s as easy as (1) inserting your Windows XP install disk, (2)restarting your computer, (3) “press any key to boot from CD,” and (4) select the partition (probably “C”) currently containing your Vista installation.
HOWEVER, if you want to play it safe, I would recommend backing up any personal data and setting up dual-boot (at least until you are sure that XP is compatible with your computer.) I would be glad to assist you with that option, if you would like, otherwise, if you could let me know the brand and model of your computer, I could possibly tell you if it’s XP compatible.
Also, there’s a (somewhat) easy way to make Windows Vista Home Premium or Basic just like XP Professional. Activating the “Administrator” account and using that account only will make your Vista machine faster, safer, and you will no longer have those extremely annoying “UAC (User Account Control)” windows that freeze your computer and ask you to allow or not allow a program to run. This is just another alternative to getting rid of Vista. Here is the best link I could find on how to activate the true “Administrator” account in Vista home: http://www.howtogeek.com/howto/windows-vista/enable-the-hidden-administrator-account-on-windows-vista/
Glad to help. Feel free to email me or add another post and let me know if you want help with the dual-boot or other options. Otherwise, it’s as easy as the 1,2,3,4 steps above!
Patrick Sewell, January 2008