I'm don't have access to my Panther machine right now to check if this works, but here's a good way to do it in Tiger...there is probably something similar in Panther.
Applications --> Utilities --> Keychain Access. In the Preferences for Keychain Access, you can click an option to show the status in the menu bar. Checking that box make a little lock appear in the menu bar. You can click on that and go to "Lock Screen" when you leave your desk, activating your screensaver. You'll need to type in your password to get back to your desktop.
dynamicv's suggestion of the hot corner is also a quick way to do it, but I apparently don't have very good aim and am always activiating hot corners when I don't want to, so I've got them turned off.