Mac OS X 10.5 Help
Locking your computer screen
If you want to stay logged in to your computer while you are away from it, but need to prevent others from using it, you can lock the screen. When you return to the computer, type your login name and password to continue working.
To require authentication to wake your computer:
Choose Apple menu > System Preferences, click Security, and then click General.
Select “Require password to wake this computer from sleep or screen saver.”
People with administrator accounts can use their names and passwords to unlock the screen.
Locking the screen doesn’t prevent other users from turning off the computer and restarting it, and then logging into their own account. If you think this could happen, be sure to save your work before you leave your computer.
To lock your screen quickly with fast user switching enabled, choose Login Window from the menu with your user name. Your applications will remain open and undisturbed, but your computer will be locked.