Here's a very simple way in which you can use a Windows Vista UPGRADE DVD (which costs less) than the Fresh Installation DVD(which costs way too much). This method exploists one small loop hole in Windows Vista. Everyone can now use Vista Update DVDs to install Windows Vista without having the operating system at hand that has to be used to update.
Type in your product key and choose a custom (advanced) installation when asked. This simply means that Windows Vista will be installed again on your system, this time with the right product key. Delete the windows.old directory afterwards which contains the files from the first installation.
You load the Vista DVD, choose not to enter a product key when it prompts for one and continue the installation by choosing the right Vista edition. This will install a 30 day trial version of Vista. Run the setup again, this time from the now fully working Vista.