Why do you actually want to downgrade? I have used both Vista Ultimate-32 and Vista Ultimate-64, currently I'm using the 64 bit edition, with my TP61p and it works excellently. For instance it's very practical to be able to see the current screens when you press the windows button and tab, if you have the aero theme running, but it's no challenge to a modern computer.
I have never seen any difficulty with my Vista. It's very evident that Vista is much more modern both in expression and when it comes to hard core computer science aspects, so why do you actually want to downgrade to the previous generation operating system?
As time goes by, Vista becomes more mature and is finally starting to work. But it has been in the last 90 days that:
1. Windows updates has fixed some wireless problems.
2. For me (and not for others), a change of wireless card make fixed more problems.
3. Adobe Acrobat standard came out with a working 64-bit product.
4. Microsoft issued a registry change to me to actually make Windows Explorer work properly.
So you see (1) I am working hard to make Vista work and (2) at this point in time (Sept 1, 2008) XP can actually do more than Vista. I keep toiling away, but I thought I would give you a reasoned answer to your question.
I will actually get Vista in production reasonably soon now.