ScreenBlur is a free tool which can temporarily lock and hide the Windows desktop.
Launch the program and it adds an icon to your system tray. Right-click this, select "Lock", and an "Access Denied" message is displayed. Type the password you chose at installation and the desktop will reappear.
ScreenBlur can also lock your screen via a customisable hotkey, or when your computer has been idle for a specified period of time.
While there are similar tools around, ScreenBlur stands out for its many configuration options. You can have your applications hidden or minimised while the screen is locked, for example. The program can mute your sound. There's an option to avoid locking "on idle" if you're watching a full screen movie. If your computer has been locked for a specified length of time then you can have it sleep, hibernate or shut down. And you're able to change the interface colours, the fonts, even set a custom lock screen image.