Open Firmware Password
Downtime can only protect the startup disk of your Mac. A saavy computer user could boot your Mac with a CD or an external hard drive and bypass Downtime. You can prevent this by adding a password to your Mac hardware (not the disk, the actual computer). When you add this extra password, called an Open Firmware Password, your Mac will only start up from the main hard drive. In order to start up from any other disk (including a software install CD or a disk utility start-up CD), you must restart your Mac while holding down the option key and then type the Open Firmware Password that you set.
The Open Firmware Password does increase the security of your Mac, but it may be overkill for most home users. If you are primarily concerned about restricting your children's access, you can probably protect your Mac adequately simply by keeping your System Installer CDs locked up somewhere.
Information about setting an Open Firmware Password can be found on in this Apple technical article.
Matterform does not offer technical support related to setting, troubleshooting or removing an Open Firmware Password.