With a support note quietly posted to its Web site, Microsoft confirmed what enthusiasts have long feared: Users who purchase Upgrade versions of Windows Vista will not be able to perform clean installs of the operating system. Instead, they will need to first install their previous OS and then upgrade in-place to Vista. Read on at WinItPro Update: Warp2Search User Cuke2u reports that there is a workaround to install an Update Version of Vista clean and smooth:
HI, actually there is a way around this if you still have a qualifying OS. What you do is clean install of vista without using a CD Key and unticking 'activate immediately'. Then you can perform an upgrade to vista as per normal and choose clean install...
We can not confirm this hint, leave a comment if you know more.