MS has provided this information in form of table, if you can upgrade your present version of Windows to Vista keeping all the apps, settings and files intact, on Upgrade Planning for Windows Vista page.
The yellow dots mean that you will need to install fresh copy of Vista from DVD, hence loosing all the settings and moreover, you will need to back up your files. It is also mentioned that if you are using WIN 2000 professional/XP profesional x64, you are eligible for an upgrade copy to a corresponding or better edition of Vista but in that case also, you will nedd to do clean install.