Before you upgrade a computer to Windows Vista, it is suggested that you use the Windows Upgrade Advisor Tool to help determine whether your computer can run Windows Vista successfully. It is also a good idea to read : How to install Windows Vista- KB918884. However if you still get Error Messages when you upgrade to Vista, here are some KB Articles which could prove to be of help. When you try to upgrade to Windows Vista, the Upgrade option that is located in the Install Windows dialog box may be unavailable. Additionally, an error message may appear in this dialog box. To help troubleshoot the issue that you experience, use the following Knowledge Base articles:

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".  KB927295.

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". KB927296.

You cannot select the Upgrade option when you try to install Windows Vista, and you receive the following message: "You must rename or remove <FolderName> before upgrade can continue". KB927297.

You cannot select the Upgrade option when you try to install Windows Vista, and you receive the following message: "Upgrade has been disabled". KB926069.

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. KB927688.

The upgrade option is unavailable when you start the computer by using the Windows Vista DVD. KB928432.

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'. KB932004.

You cannot select an upgrade option for Windows Vista, and you may receive an error message: "You cannot upgrade Windows in Safe mode". KB932005.

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". KB932006.

Error message when you try to upgrade a computer to Windows Vista: "The installed operating system does not support an upgrade to Windows Vista". KB932007.

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". KB932009.