Here is a complete list of Errors that you can possible get when upgrading to Windows Vista. I have given a brief one line explanation of what the problem could be and have linked to the relevant KB(Knowledge Base) article that gives more information on the same and how you can easily fix the error. Hope this helps you during the rather painful upgrade procedure.
927295 - You cannot select the Upgrade option when you try to install Windows Vista, and you receive the following message: "Windows cannot determine if another operating system exists on your computer"
927296 - You cannot select the Upgrade option when you try to install Windows Vista, and you receive the following message: "Your upgrade partition has multiple operating systems"
927297 - You cannot select the Upgrade option when you try to install Windows Vista, and you receive the following message: "You must rename or remove before upgrade can continue"
926069 - You cannot select the Upgrade option when you try to install Windows Vista, and you receive the following message: "Upgrade has been disabled"
927688 - You cannot select the Upgrade option when you try to install Windows Vista, and you receive an error message that is specific to hardware that is installed on your computer
928432 - The upgrade option is unavailable when you start the computer by using the Windows Vista DVD
932004 - Error message when you try to upgrade a computer that is running Windows XP to Windows Vista: "The legacy OS does not meet the software update requirements: 'Service Pack 2 for Windows XP'"
932005 - You cannot select an upgrade option for Windows Vista, and you may receive an error message: "You cannot upgrade Windows in Safe mode"
932006 - Error message when you try to upgrade a computer to Windows Vista: "The upgrade cannot be started because the content on your installation disk is written in a different language than the language on your computer"
932007 - Error message when you try to upgrade a computer to Windows Vista: "The installed operating system does not support an upgrade to Windows Vista"
932009 - Error message when you try to install Windows Vista by using Windows Vista installation media: "Upgrade has been disabled - To upgrade, start the installation from Windows"
927295 - You cannot select the Upgrade option when you try to install Windows Vista, and you receive the following message: "Windows cannot determine if another operating system exists on your computer"
927296 - You cannot select the Upgrade option when you try to install Windows Vista, and you receive the following message: "Your upgrade partition has multiple operating systems"
927297 - You cannot select the Upgrade option when you try to install Windows Vista, and you receive the following message: "You must rename or remove