If the account name is "Administrator", it's the default administrator account and can't be deleted, as Allan said.
You can have multiple administrator accounts on a single machine. From the main administrator account, go to User Accounts in the control panel. Select your normal account, click "Change account type" and switch it from "Standard user" to "Administrator".
Any other administrator accounts that you do not want can be deleted, except for the Windows default administrator account. To delete other accounts, click "Manage another account". From this next screen, you can enable or disable the guest account, and delete any other accounts. To delete an unwanted account, click the account then click "Delete the account". You will be given the option to either delete or keep that user account's files which will save documents, favorites, music, pictures, videos, etc. into a new folder on your administrator account's desktop. All email messages and other settings from the deleted user account will be lost.
