For example, i'am on computer who' is protected by a password on Administrator, and Guest account is on.
I loged on to Guest account and how can I put the Guest Account in Group Administrators (with admin privileges)?

On XP it's possible.. on vista no



Sorry for bad english...

In vista you won't be able to add an account to the admins groups without being an admin on the machine, there is no command to add the guest to the admin group logged on as the guest

If you have physical access to the machine you can create a bootable USB pendrive with Petter Nordahl's software to promote the guest account to the admin group temporarily.

http://home.eunet.no/pnordahl/ntpasswd/

This method works the same for NT5 and NT6 but inside the running machine with guest permissions without physical access or an account with admin permissions to the box it's very difficult to overcome all the present security restrictions as fizban2 mentioned.