Well what might work is unplug it from power take the battery out and hold down the power button for 30 secs it should be a reset if that dosent work do the same thing but also remove the bios/cmos batter from inside the laptop and that should clear it
here is a website with a list of backdoor passwords you can try they will let you bypass and change what you already have
or you can call DELL and give them you serial number and they can get a backdoor password that will bypass what ever you have now