Windows Media Player won't show DVD video when laptop is unplugged

Windows Media Player won't show DVD video when laptop is unplugged

I have a Dell Inspiron Mini 10 laptop running Windows XP. Even though it doesn't have a DVD drive, using an ISO imaging tool I can mount DVD images as local drives. I can then play DVD movies just fine using Windows Media Player, as long as the power cord is plugged in. If unplugged WMP will only play audio and not video.