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Uninstall Windows Vista using Uninstall a Program
http://www.vistaarticles.com/articles/290/1/Uninstall-Windows-Vista-using-Uninstall-a-Program
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Published on 12/10/2006
 
After you upgrade, you cannot uninstall Windows Vista by using the Uninstall a program option of the Programs and Features Control Panel tool.
Uninstall Windows Vista using Uninstall a Program

To resolve this problem, back up your files, and then perform a clean installation of Windows XP. To do this, follow these steps:

1. Click Start, click All Programs, click Maintenance, and then click Backup and Restore Center. 
2. Click Set up backup, select the location for your backup files, and then click Next. 
3. Continue with the wizard to select the following:• The drives and the file types to back up
• How frequently to back up the drives and the file types
• What time of day to perform the backup
 
4. Finish the wizard, and then click Save Settings and Start Backup. 
5. When the back up is complete, perform a clean install of Windows XP. For more information about how to perform a clean installation of Windows XP, click the following article number to view the article in the Microsoft Knowledge Base:
316941 (http://support.microsoft.com/kb/316941/) How to install or upgrade to Windows XP 

Note You cannot perform a clean installation of Windows XP by using an upgrade version of Windows XP. You must have the full version of the Windows XP.