It's not as simple as clicking on it and finding the control for this, but it's not that far off either. This configuration has to be accessed via the control panel. Apparenly what you're doing is changing globals that apply for all software on the machine. Here's the link with nice screensnaps:
http://answers.microsoft.com/en-us/windows/forum/windows_vista-desktop/control-panel-change-12-hour-clock-to-24-hour/490a874b-7d3a-4fa4-ad38-c6ebc24a854a
