Your English is fine, no need to apologize.
The correct way to describe your cables is cat5. As I understand it, you have two cpus connected to a router with cat5 and the router connected to a broadband modem.
Make sure that your network connection is good before you try to open the shared folders by pinging the other machine first. Go to a command prompt (I'm not sure how to do this in XP) and type "ping" and then the ip address of the other machine. If it works, your pc is probably the problem. If not, you're dealing with buggy software and I would uninstall and reinstall all the network services you're using. I think you've indeed set up all the necessary networking protocols, and they just aren't working for some reason.
“The highest glory of the American Revolution was this: It connected in one indissoluble bond, the principles of civil government with those of Christianity." -John Quincy Adams