XP + Pentium-M / Core-Duo + USB 2.0 = Short Battery Life
Anandtech have posted a really interesting article about the Windows XP USB 2.0 power bug. Check it out here.
Basically if you have a Core Duo or Pentium-M processor laptop with a USB 2.0 device connected and you're running Windows XP then the chip cannot enter it's low power states. That results in about a 15-30% reduction in battery life. Surely you can just disconnect the device? Not in all cases. Some of these laptops have built in USB 2.0 cameras. Oooops. There is a 'fix' out there but it's not official as yet. If you own a Pentium-M or Core-Duo laptop I advise you to check out the article for more information.
Microsoft have released a fix.