I am glad that I received so many excellent answers from our friends.
I bought a switch from a shop in town and tried. It didn't work. I returned it.
I went to another shop and bought the following switch and it works.
USB 2.0 Peripheral Switch.
Enables 4 Computers to Share a Single USB Device
Model US-421
If you run windows there is a CD to install drivers.
I don't run windows.
http://www.aten.com
This is a Taiwanese company.
You just press the 'Num Lock' button twice to change the computer.
It works fine.
There are some buttons on the switch to change the computers.
I don'ẗ need those buttons.
Just press the 'Num Lock' button twice to change the computers.
Sometimes the keyboard hangs when starting computers.
What is the problem?
As I mentioned before I am running 2 computers with one Samsung monitor.
I bought a switch for this purpose nearly 3 years ago.
This switch works just pressing 'Scroll Lock + Enter' buttons.
I know this switch is rotten to some extent.
It may be the keyboard is not capable of handling so many switches.
Do the keyboards get old?
I am clueless on these things.
When the computer starts, the system forced to recognize 2 switches.
Your thoughts are welcome.