unwanted autostart of pentium II

i am a pc-recycler and have several pcs opened beside the monitor. even without cordless mouse attached one atx-pentium II sometimes starts unwanted. one time when i used tv-remote control(ir-mouse was installed), the other time when i worked on my cable-modem-pc). one time second pc was on but i did not work. i will check if there is a suspend-mode activated. the power-modul has no switch so i unplugged the cable.