If you are running XP x86 you cannot upgrade to Vista x64. If that is what
you are trying to do, you cannot. You can only install Vista x64 with a
clean install. One reason for this is that x64 requires signed 64-bit
drivers.
The upgrade advisor is there to provide a to do list of things you need to
take care of before installing Vista. It does not mean that you can upgrade
from one beta build to another easily.