What is the metal key at the back of the PC used for?

Hi, @littlemisspro, welcome to Fluther!

Probably the key will have no effect on running the computer. Or it is an on/off switch.

Most likely it’s a lock to prevent tampering with the insides. In a corporate setting, you would want to prevent people from taking memory home or making any changes which might cause headaches for the IT department.

Or, it may be an off/on switch, so you can prevent people from running the PC without the key.

But I think that is less common.

Probably to secure the components if the box were taken to a LAN party or if it were left in a public place like an Internet cafe (do you remember those?!!).

Welcome to Fluther.

It’s a system lock, designed to prevent unauthorized use of the machine. It should fit only one way, and if it’s not installed (or not oriented properly) then the system simply won’t boot up.

Keep trying, and you’ll see how it fits and which way it needs to turn.

Thanks :D