|The term CPU is frequently mis-used|
riider has pointed you to what a CPU actually is.
The computer case that most if not all your computer's components are installed in IS NOT A CPU - it's just a computer case.
I'm guessing from your poor descriptions that...
- you have a desktop computer, not a laptop or netbook computer
- the power supply installed in the computer case has a power switch on it, on the back of the computer case.
- when you plug the AC power cable, that is plugged into a source of live AC power on the plug end of the cable, into that power supply's socket for that on the back of the computer case, if the power switch on that power supply on the back of the case is switched off when you do that, the computer does not start up.
- in that situation, when you then switch on that power switch on that power supply on the back of the case, the computer starts up automatically.
How your computer behaves when AC power has been restored to it's power supply, when the switch on the power supply itself is switched on, whether it's when it's AC cord is already connected and has live AC power when you switch on that switch, or when that switch is already on on the power supply and you plug in the AC power cord that is connected to live AC power, depends on a setting in the mboard's bios Setup.
In that case, that setting in the bios is set to start up the computer when AC power has been restored.
However, the computer MAY start up automatically in any case even when that setting in the bios Setup has been set to NOT start up the computer when AC power has been restored, if switching on the power switch on the power supply itself, or plugging in the cord into the power supply from a live AC source when that power switch on the power supply itself it already on, or even when you switch on a power switch on something the AC power cord is connected to on it's plug end, if doing that produces electric or electronic noise, which the mboard interprets as you having pressed the power button for the power switch on the front of the computer case.
Look in your bios Setup to see if a setting is set to start up the computer when AC power is restored.
If it is set that way, if you don't want the computer to do that, change that setting, Save bios settings.
If it is already set to NOT start up the computer when AC power is restored, then your computer starting up automatically is caused by electric or electronic noise being produced by whatever you did.
As long as you don't do whatever caused the electric or electronic noise to be produced by whatever you did, when the power supply is getting live AC power and it's switch on, on it's own switch, the computer will not start up unless you press the power button on the front of the case.