i have a compaq pc with windows xp installed on it. lately a strange siren like sound has started coming from my pc. sometimes it keeps coming until i restart while sometimes it comes for a few seconds and stops. if i reduce the speaker volume to zero, it reduces but is audible but if i increase the volume it becomes louder. also when it comes is not fixed and whenever it comes the pc sort of hangs for a few seconds. does anyone know what is it?how can the problem be fixed? please help.