The purpose of having a logon process is to establish who you are. Once the operating system knows who you are, it can grant or deny requests for system resources appropriately. If a bad guy learns your password, he can log on as you. In fact, as far as the operating system is concerned , he is you. Whatever you can do on the system, he can do as well, because he's you. Maybe he wants to read sensitive information you've stored on your computer, like your e-mail. Maybe you have more privileges on the network than he does, and being you will let him do things he normally couldn't. Or maybe he just wants to do something malicious and blame it on you. In any case, it's worth protecting your credentials.
Always use a passwordit's amazing how many accounts have blank passwords. And choose a complex one. Don't use your dog's name, your anniversary date, or the name of the local football team. And don't use the word password ! Pick a password that has a mix of upper- and lowercase letters , number, punctuation marks, and so forth. Make it as long as possible. And change it often. Once you've picked a strong password, handle it appropriately. Don't write it down. If you absolutely must write it down, at the very least keep it in a safe or a locked drawerthe first thing a bad guy who's hunting for passwords will do is check for a yellow sticky note on the side of your screen, or in the top desk drawer . Don't tell anyone what your password is. Remember what Ben Franklin said: two people can keep a secret, but only if one of them is dead.
Finally, consider using something stronger than passwords to identify yourself to the system. Windows 2000, for instance, supports the use of smart cards, which significantly strengthens the identity checking the system can perform. You may also want to consider biometric products like fingerprint and retina scanners .