Default sound volume on Windows XP startup

For the last few months, regardless of where I leave the volume setting at shutdown, when Windows XP is restarted it does so at maximum volume and plays the Windows Startup sound VERY LOUDLY and wakes everybody up! Once all startup applications are running I can reduce the volume using the standard Windows Volume control, but it always defaults back to 100% on restart. This is really frustrating me, disturbing our sleep and taxing my speaker diaphragms to their limit. Anyone know what has changed and how I can restore the previous setting where the startup volume was the same as the shutdown volume?