What basic security rules should I follow to ensure the protection of personal data temporarily stored on my personal computer?

Without prejudice to others, the following rules must be ensured: The password for accessing the computer must be at least 8 characters long, it must be memorisable so that it does not have to be written down elsewhere, but it must not be the result of one word or a string of words. The computer must have all the latest security updates. If the computer contains personal data, its use should be restricted to third parties.