The procedure is likely different under Windows XP than the operating system installed on your older system. Click Start | [Settings] | Control Panel | Regional and Language Options. Then, next to your current Standard and Formats settings (which should be English (United States) within the US), click the Customize button. Click the Time tab, and select your Time format as HH:mm:ss. Click Apply, then click OK. Click OK again, and your system should now display the time in Military format within the system tray.