CPU usage spikes

Need help !!! has anyone encountered in Dell D600 that no application is running on the desktop but still the CPU usage is spiking as in CPU usage is ranging from 10% up to 40%,  like its making small mountains :) , pls advise me if there is something wrong with hardware or software....but i've tried reformating my O.S. still problem occurs or any patches needed                                                                                                                           

                                                                                                                        thanks