I can see there's a little confusion here... Windows XP x64 (i.e. 64bit OS) and Windows Vista are two completely different things.
Today, there are 2 version of Windows XP - 32bit and 64bit. When Windows Vista is released, there will also be 2 versions - 32bit and 64bit (well, it seems there'll be
many versions of Vista, but we're talking about the "number of bits" right now).
Now, the boot-time scanner supports just 32bit NT-based platforms right now. So, it works under the "ordinary" (32bit) Windows XP, and it also works under 32bit Windows Vista (even the current beta versions, I hope). Right now, 64bit platforms are not supported. I hope it will change in the future (the
non-Intel versions of avast! engine are the first step towards it, but it will probably take some time.