When you start your Windows XP laptop using battery power, the processor speed may be slower than the manufacturer rated speed.
To view the processor speed, look at the General tab of the System applet in Control Panel.
This behavior occurs because most manufacturers run the processor at a slower speed when the computer is running on battery power, to reduce the heat and conserve battery power.
NOTE: This behavior may be configurable in your computer's BIOS.
When the computer starts, the processor speed is written to the ~MHz Value Name, a REG_DWORD data type, at HKEY_LOCAL_MACHINE\HARDWARE\DESCRIPTION\System\CentralProcessor\<Processor Number>, where <Processor Number> starts at 0.