if it is restarting before OS is booting , then check the smps output voltage.If it is ok then disconnect all pheripherals(cd rom ,hdd,add in cards). Only the processor and ram should be present.connect the monitor also.now boot and check whether the system is restarting,If so then take the ram and processor and clean it and fix it . If the system still not booting ,touch the south bridge co processor . If it is too hot then mb is problem. otherwise touch the processor heatsink if it is too hot then processor is problem.