Vista is the code-name for Microsoft's next-generation Operating System (previously called Longhorn): the successor to Windows XP (which was code-named Whistler). It is still a long ways off, a couple of years. But you will begin to see alpha and beta copies floating around the Net.
The best way to get a copy is to become a beta tester [sign in using the guest account 'BetaReq' (case sensitive)], or know someone who is. My previous contact has since dropped out of Microsoft's beta-testing program. He used to give me his log-in and password, so I could download the latest revisions directly from M$ (fast, secure download).
I keep a partition dedicated on my system so I can always play with the latest OS-toy from M$. Unfortunately, I have not been able to find a copy that downloads faster than 1-KBps, which would take 100 years to download.
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The best place to learn about Vista/Longhorn is at Paul Thurrott's WinSupersite. Paul gets into some detail here about build 4051. Here is the official M$ Longhorn developer's site. Microsoft has also recently added a page specifically for Windows Vista. Wikipedia has a nice, concise introduction to Longhorn posted here. Information Week discusses it here.
The *next* operating system after Vista will be based on (code-name) Blackcomb, which is named after a mountain in British Columbia, not far from Redmond, where the Microsoft headquarters are located. Whistler, upon which Windows XP was/is based, is also named after a mountain in BC.
So is Longhorn also named after a mountain in British Columbia? No. It's actually named after a saloon located at the base of Whistler mountain, between Whistler & Blackcomb. The story is posted here (200-KB PDF from the Vancouver Sun).
This page updated 24.July.2005 to reflect the name change from Longhorn to VISTA.
A Google search for "windows longhorn" is found HERE.