how to avoid privilege escalation

I'm an admin, for my concern

One of the worker, has escalated his privilege from guest to admin.

How to solve it?
How he done this?

with regards,
Nirmal jose

He needs to know the password to an administrator account to do this.

HOw he knows, my password?

Its worlds best, complex password....

could you can step me, how this done???/

Could you can give me, 10 mail id of information security
professionals , by now (in India)

net user <username> *

this commands, does not works in GUEST account, why??/

HOw to overcome this???

I don't know how he knows your password, or the password to *another*
Administrator account. But I can tell you that you *cannot* logon as a
guest (or other limited user account) and escalate or grant yourself
privileges to administrator level, only administrators can do this. Is
this on a domain or on a stand alone machine? If it's a stand alone my
guess is that the built-in administrator account is unprotected, or it
was hacked.

No, I can't.

Log on with an administrator account! If you are trying to do this from
the Guest account and if you are asking for a workaround to the
restrictions imposed on the Guest account I can only surmise that you do
not have permission to do these changes on your machine and that you are
trying to circumvent restrictions imposed on you by your IT department,
ask your boss to hack the account for you...