Traditionally, you are required to have a previous installation of Windows before you can upgrade, e.g. have Windows XP on your system before using Windows Vista Upgrade DVDs.
What if you have ALREADY upgraded to Windows Vista but are having system issues that require formating and re-installing Windows? Paul Thurrot proposed this hack that lets you install a Windows Vista Upgrade, without needing to re-install Windows XP first:

1. Boot from the Windows Vista Upgrade DVD and begin the setup program.

2. When prompted to enter your product key, DO NOT enter it. Click "Next" and proceed with setup - this will install Windows Vista as a 30-day trial.

3. When prompted, select the edition of Windows Vista which you have purchased and continue with the setup.

4. Once setup has been completed and you have been brought to the desktop for the first time, run the install program from within Windows Vista.

5. This time, type in your product key when prompted.

6. When asked whether to perform an Upgrade or Custom (advanced) install, choose Custom (advanced) to perform a clean install of Windows Vista.
(Yes! This means that you will have to install Windows Vista for a second time)

7. Once setup has completed for the second time, you should be able to activate Windows Vista normally. You can also delete the Windows.old directory which contains information from the first Windows Vista installation.

This means that all Windows Vista upgrade DVDs can be used as full retail DVDs. However, this is an illegal approach and you are going against Windows Vista's Eula should you install an upgrade without owning a previous version of Windows as specified by Microsoft.

Are you sure,dude?

From my info, Its an illegal method...but still it is an overlookable offence!

;)

Yep. Its LEGAL.