Sentry Professional Description:

Sentry locks down your computer to keep it secure when you aren't around. Many options are provided - you can customize the appearance of your screen, have your favorite screen savers run from within Sentry, and even take messages from people who may drop by.

Sentry can be activated instantly. It can be launched after your system is idle for a specified period of time, by clicking on an icon in the system tray, or by typing in a set hotkey.