How do I find a back door password on my computer?

It depends what you mean by "backdoor" if you mean "how do I change my password", than use cmd.

Explanation:

CMD (also known as Command Prompt) is a very powerful program used for Windows (assuming your using a windows). To change or disable any account on your computer use the following commands...

Change Password: net user <username> <password>

PRESS ENTER

Disable/Enable an account: net user <username> /active:<yes/no>

Be aware that if you disable all accounts on your computer you will not be able to access your computer. So use this command with caution. Should you decide to use command prompt, and you cannot gain access to an account, just go into recovery mode, and turn an unactive account active. Should you lose a password to your account, the recovery mode for your computer can work with this too.

If this does not answer your question, please let me know, and I will help you out.

Remember that there is no true way to protect your account without downloading 3rd-party programs that monitor your computer. But using cmd if someone gains access to your account, will always work. This will also allow you to gain access to the built-in Administrator account that if you have any problems on your main account, the built-in Administrator account should be used to fix any said issues.