how can i delete a administrator user account????

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i have window media centre, in the user account by mistake i have 2 administrator account 1 by default(named administrator) 2nd named vishal(that is also a administratot account). now the problem is that i am not able to find the administrator account, when i open the computer there is only vishal & guest. so the folder of default administratot i can't get.i also want to delete this vishal. so pls help.