If you want to set a 24-hour clock (military time) on your computer, here's how to go about it.

For a Mac, click on the time in the menu bar and select "Open Date & Time...". This will open the Date & Time window in System Preferences. Select the Clock tab and then check the box next to "Use a 24-hour clock."

For a Windows machine, either on XP or Vista, go to the Start menu, choose Run and then enter intl.cpl and hit Enter. This will bring up the Regional and Language Options panel in the Control Panel. From the Regional Options tab, select the "Customize..." button if you are in XP, for Vista select the "Customize this format" button. Selec the "Time" tab in the new panel that opens. There will be an entry called "Time format:" in this panel. Select that dropdown and choose the HH:mm:ss option and then press OK.