[i]
also, i've found another weird account : SYSTEM.

when you start the schedular for a taskmanager, then everything you open in the TASKMAN is run as system, so if you open the explorer.exe in the TASKMAN then you will see that you are system when you goto start.
but not possible to login with this account!

any ideas? [/B]
The System is not an account that you can log in as. It is an account that runs most of the processes that windows require and are started automatically. It is found in 95-XP M$ OS. The System permission is also needed on many files and folders for the workstation or server to function properly. For example, try removing the system permission from all of your folders and files and try to run a Symantec Antivirus scan, as of SAV 8.0, the scan will run but since it uses the system credentials it won't have access to any folder or file. Many pieces of software use the system credentials. Likewise, most virus coding use the system credentials, since by default the system has unlimited access. So you may now ask how to keep your system account from being abused.
1) Partition your hard drive in at least 2 parts. Load the OS on 1 and the rest of your files on the other. this is good practice anyway.
2)Decide which folders have to have the system permissions, in reality, you don't have to have the system permission on any, but then we get back to the SAV part above.
3)Stay current on your OS security patches.