Actually I too faced same problem with my pc
Actually once we set password and choose it for both bios,booting
then only it asks password when you start the computer
F12 is nothing but boot menu means we can choose which device must be started first means like first boot device cdrom
second boot device HDD like wise

you have to continuasly press DEL key at "Acer" screen it will ask you to enter password because it is enabled then you'll see a blue screen with some facilities
there you'll find Set supervisor password then in there you'll find administrator password and then there you'll find all the required actions according to your desire

You must find the option just like
1.Set supervisor password
or
2.Security Settings

In my motherboard bios it is Supervisor password i don't what's in yours is???