when I unplug my netbook, I can use my keyboard. I can write my password on login screen, but nothing on the softwares (firefox, mail etc...) I must close and open the session for my keyboard is OK I'm on eos 5.1 (Atom N450, 2gb of ram, model ACER D255)
This could be a USB power issue.
Depending on what kind of keyboard you have, it's possible that it's drawing more voltage than your Netbook can handle when not plugged in.
You might consider using a powered USB hub. I like the ones Plugable provides since they were recommended to me a while back as being unusual in that they provide really solid power (not always a given among hubs apparently.)