DesktopKeeper

If you want to leave your computer without shutting it down and being sure that no one will touch it or access your desktop while you are not present, then this application might be really useful for you.

What DesktopKeeper does, is blocks momentarily your desktop so that no one without a password can use it. If someone shows interest in using your PC while you are absent, the program will let you know about that when you come back.

DesktopKeeper was specially designed for Windows 2000 and Windows XP, but it also works on Windows 95, Windows 98 and Windows ME. The application comes with configurable hotkeys to lock the system in a very short amount of time.
There are different ways to lock the desktop. You can right click on the application's icon, add a password and then hit the lock desktop button or you can also lock it at a specified time. All you have to do to unlock the desktop is to right click on it and enter the password. Once you've done that, you get full access to your desktop as if you had never used this program.