My computer has HD screen, hardware, HDMI port etc but will not play HD movies.

My laptop, which was ordered about a month before windows 7 came out, has windows vista and everything is set up for high definition viewing and sharing. I know this for a fact because I paid extra (not just a small amount extra) to have the computer with the HD screen and components. The HD movie I downloaded from the Itunes store will not play on the laptop because "it's not high def" according to the message. Am I missing something, or are the movies set to only work with Windows 7 and nothing earlier?