XP

KB 931321 - Battery power may drain more quickly than you expect on a Windows XP or Windows Server 2003-based portable computer.

It specifically fixes this scenario:
"The portable computer cannot enter the deeper Advanced Configuration and Power Interface (ACPI) processor idle sleep states."

"This occurs when the USB 2.0 driver leaves the asynchronous scheduler component running continuously. This prevents the processor from entering the deeper ACPI processor idle sleep states, or C-states. C-states save power when the processor is not busy. These states range from C0 to C3 or C4. Typically, an idle portable computer uses the C3 and C4 states to preserve battery power. If an idle portable computer cannot enter or maintain the C3 or C4 states, the battery power drains more quickly."

You will typically get better battery life if your laptop can enter into the 'deeper sleep' C-states.

Check out these crazy (in a good way) underclockers from Germany here. I've done some crazy experiments in the past like this, but not to that extreme. A lot of people wonder why someone would want to do something like this, but I understand why they do it - because they can.

You can learn a lot of tips and tricks by doing experiments like that.