The Customer Preview Program has ended, meaning you can no longer download the beta and RC versions, which were free of charge. Also note that Microsoft has forbidden others from 'sharing' the ISO as product keys, so anyone offering such would be breaking the law.

Now, Vista went RTM (released to manufacturing) on November 8th and is expected to hit MSDN on the 17th, so if you have an MSDN subscription you can download it from there. Otherwise you'll have to wait until you either purchase a new computer or can pick up a boxed copy in a store starting around January 30th. (Microsoft does not offer released OSes for download to the general public, though you would be able to order a DVD online at that time.)

John