Cannot re-set System password

I have an Inspiron 1525, and previously had both Administrator and System passwords set.  I removed the System password so that the laptop did not require a password on bootup, but now want to re-set it.  I have tried leaving the Administrator password and unlocking the password setup, but the laptop will not accept a new System password (I have tried several, all over 4 characters in length).  If I remove the Administrator password it will let me set a System password, but (as intended) it will then not let me re-set an Administrator password.  I have tried removing both passwords, rebooting, setting an Administrator password, unlocking password setup then trying to set a System password, but it still won't work.  Please can anybody help?