You'll need to logon to the administrator account then.
Press Start > Log off > log off
Now in the user menu, you might only see your account and the other, but not administrator.
Hit alt+ctrl+del twice to access an advanced logon option.
In the username part, type in "administrator" without the quote and try to logon.
If it won't let you because of a password, try admin or administrator.
If you can get on, go to Start > Control Panel > User account.
Choose Seth if it's there and see if there's an option to delete it.