RE: Power adapter pins
The password that is showing up before your computer boots sounds like there is a BIOS password set on the computer. Unfortunately if you don't remember that password then you are probably stuck. You can probably go online and search and see if there is a way to reset BIOS on the laptop and get it to default and remove the password that is set on it. If there is a hard drive password though then the only way to remove that if you forgot it is to install a new hard drive as the password will follow the drive, not the computer.
Your power supply should have only 1 pin. Dell used the same adapter for their computers for a while, they come in different wattage though and if you use an adapter that is too low then it may not be able to charge the battery while you use it or at all. If the plug fits and it is a Dell charger you should be okay to at least get the computer up and running. If it is a low wattage charger and the setting is set in BIOS (usually is by default) you'll get a message from your computer telling you that the charger isn't powerful enough to charge the battery while in use.