Microsoft offers retail upgrade edition of Windows Vista as a cheaper option of existing users of Windows operating system such as Windows XP to upgrade to Windows Vista without paying the full price. However, the Vista upgrade DVD or upgrade key requires users to first install their previous version of Windows OS and then upgrade in-place to Vista. With this trick, you can buy the ultimate upgrade edition ($249.99 from amazon.com) and install it like the full edition ($379.99).
The method is very simple.
Boot up your computer with Windows Vista Upgrade DVD.
Click on “Install Now” to start the Windows Vista installation process.
Do not type or enter any product key when setup asking for product key for activation. Click on “Next” button.
When prompted, select the Windows Vista product edition that you have purchased or owned. Continue to complete the Windows Vista setup process. Once Windows Vista has been installed, boot up and log on to Windows Vista. Put in the Windows Vista upgrade DVD into DVD drive, and restart the Windows Vista setup again, but now from within Windows Vista (Vista should auto launch the Install Windows welcome screen or dialog).
When prompt to type your product key for activation, enter your valid and genuine product key for Vista. When prompted for option to choose which type of installation do you want, select “Custom (advanced) to install a clean copy of Windows Vista, and do not keep any files, settings and programs.
Select Custom to install fresh Vista during second install of Vista setup. Complete the installation and you will have a clean and fresh Windows Vista when done. Files from first Windows Vista installation will be moved to Wndows.old directory that you can safely removed.
Activate Windows Vista as per normal.