Because of high CPU usage by the lsass.exe process I googled then discovered the guest account is enabled. I disabled it yesterday & rebooted, as the result CPU usage became normal.
Today I checked the things again and found Guest is disabled, at Task Manager it says Guest disconnected and lsass.exe gone crazy. I forced to logoff Guest. CPU is ok again.
What could cause this?
If the server is a member of the domain, make sure to disable both the local guest account on the server (possibly via group policy) and in active directory.