CD-ROMS actually consume more power than a laptop hard drive. I think it's mostly because of how highly inneficient lasers are. So I think you would actually burn your battery up faster using a boot cd instead of your HD.
There is something that you can do, though. I read in a skymall magazine that there is a small generator that you can buy (I think about 100$ USD) that is powered by repeatedly pressing your foot down on a pedal. You can get up to 20 minutes of power for about a minute or so of this. I don't know much more about it, but if it's that much of an issue to you maybe you should check it out

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