Question: I am in a position where I might have to prove that I made a certain amount of MB downloads from the Internet including webpages browsed to a court of law. Is there a way of finding this out from Windows Vista Ultimate. If I need to do this in the future is there a way of setting up Ultimate to audit my account to provide a trail of downloads ?

Answer: 
I am not a lawyer... but from what I know of the Windows auditing sub-system, you're not going to be able to track downloads. The particular browser (IE, Firefox, Safari) you used likely contains history that will show these downloads.

What will probably be of more value is working with your Internet Service Provider (e.g. Comcast, Qwest, Verizon, etc.). They likely will have logs that indicate what sites you were accessing, when and what was transferred.

Regarding the longer-term solution (where you want to be able to prove that you downloaded something) this can be difficult as any system that you own and control can probably have relatively easily faked logs (and may not really be trusted). Using a third-party proxy site that logs these access may help as it shifts the record keeping to someone else.
Your ISP should have a log with your visited sites and all of your network transfers. Try contacting them.