UltraHide

From XetoWare:

If you are worried about doing something you shouldn't be, somewhere you shouldn't be, worry no more because there's UltraHide. UltraHide is a free windows hide tool that lets you hide any program that is running on your screen. By pressing a combination of keys, you can easily hide your current window from sight and later retrieving it from the taskbar tray. However UltraHide doesn't stop there. UltraHide takes a step further in hiding windows by password protecting them. In order to show windows hidden by UltraHide, computer users will need to know the password that was set beforehand. You can fearlessly use a computer with UltraHide by your side.