Frederico Fonseca:
thank you very much for all your help, i allready found the problem,
the situation was that appears an ip address duplicated out of my building, but something strange still happen, when i found the switch which contain the pc with the problem, it was unpluged, and then try to find out personaly which of the pc's has the duplicated address. My surprise was that all of the Pc's don't have the ip address duplicated, and i still continue seeing alive the ip address duplicated, so until i reboot the switch the problem disappear.
i was thinking in a virus, but i'm not sure, i still monitoring my network and i hope to find the problem early.