My computer sometimes makes a sound, even when I set it to mute?

Hi, I modifyed the sound settings on my Windows XP to no sounds at all,... but sometimes, I hear the “Windows XP start” sound like 3 or 4 times in a row, even when the computer is doing nothing, and nothing is open… I don’t know where this may come from, maybe a software does it, or a setting…. what could it be?