EEPROM on laptops
On laptops, the password is usually stored in an eeprom on the motherboard and not in the cmos. You need an eeprom programmer/eeprom reader (electronic device) to retrieve the password. If you erase the cmos (ie. cmospwd /k) and if the password is really stored in an eeprom, you won't be able to boot anymore.
You can get/buy eeprom programmer in electronic shops or labs, you need another PC to use it. You can desolder the eeprom with hot air or you can try to "clip" the eeprom. With the eeprom programmer, backup your eeprom and run cmospwd /d /l eeprom_backup. If you don't see the password, you can try to fill the eeprom with zero or FF, don't forget the reset the cmos.
HP / Compaq
HP NX9010 24C02