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: 
It completely depends on what you are using to download. Can you give us a bit more information about your environment? What browser(s)? Do you use download managers? What kind of downloads are you talking about? etc...

Your best bet is to set up some kind of logging proxy to run your Internet traffic through. This would record all of your Internet traffic and provide raw data for statistical analysis (sites visited, files downloaded, size of downloads...).

But even then, data in these types of systems are easily forged...
Your ISP should have a log with your visited sites and all of your network transfers. Try contacting them.