The beta for Kindle for PC will be available as a free download in November. You can sign up here.
Kindle for PC will work on earlier versions: Windows XP and Windows Vista.
Kindle for PC can also be used on handheld devices such as this HP TouchSmart tx2. Windows 7 features such as Windows Touch for zooming in and out with a pinch of the fingers and turning pages with the swipe of a finger will work on Kindle for PC.
You can read Amazon's Kindle e-books on Kindle for PC.
And of course you can buy Kindle e-books and access them on any PC.