If you are away from the computer either for any reason, you should lock the system for safety reasons, especially if you are in office or in a public place. Here are some tips without the use of any software or any other application.
Through the keyboard:

This is the easiest way to block Windows XP, simply press the Windows key and the letter "L".

Track cuts:

If for some reason failed to go through the keyboard, there another way to lock the operating system. All you have to do is create a shortcut by right-clicking the desktop and choose shortcut. As shown in the image

Just copy and paste this code 'rundll32.exe user32.dll, Lock Work Station' and next;

You can name it anyway, in my case I blocked names to make it more recognizable.

Through the command line:

This step is similar to above, all you have to do is copy the same code on the command line. Open command from the Start menu or click run and type CMD and then copy the code rundll32.exe user32.dll, Lock Work Station.

As you can see there are various ways to block the Windows XP operating system. However, it is preferable to use a password and lock the system would be useless if everyone can join just by clicking on the user account without typing a password