You can install any version of Windows Vista without a product key.

The Windows Vista retail DVD contains every version of Windows Vista.

When prompted to enter a valid product key, don't enter anything in the textbox.

When you receive the Are you sure dialog, pressing No will present you with the list of Windows Vista versions.

After selecting a product from the list, you may use it without any problem for 14 days,after which you must enter a valid product key, or do a fresh install. This makes it easy to evaluate the different versions before you purchase.