If you want a modal dialog box (a panel on top of all other CVI panels), use InstallPopup(panelHandle) instead of DisplayPanel(panelHandle). Other Windows apps may be displayed on top of all your CVI panels.
If you want the CVI app to always be on top of other Windows apps but still allow access to other Windows apps, set your main panels Floating Style to Always. In the UI editor, double-click on a blank spot on your main panel, click on Other Attributes, then select Always under the Floating Style.
If you want to deny any access to other Windows apps while your CVI app is active, you need to use Windows API calls to create the window as a system modal dialog. You'll have to create the entire user interface through API calls. It sounds like a headache to me.