After uninstall Avast8 on Windows 8(I do not have other OS, so I do not sure if this problem also appeared on other OS), Windows tell me there is an unknown network devices(WAN Monitor).
After a simple annalyze, I suppose that the avast uninstall wizard may have delete the Firewall Driver but did not delet the registry key for this driver, since I found the registry key for this driver and the Avast NDIS Protocol in the Property menu of network connections after my uninstallation,but I did not found Firewall Driver.