What do you mean by registered computer?
When you register a computer, we assign a unique identifier to that computer that only we can recognize. This eliminates the need to answer a security question whenever you sign in from that computer.
Typically, you would register the computer you use most often to sign in to online banking, such as your home or work computer. However, you should refrain from registering a computer that can be accessed by the general public, such as a computer at a public library.