how do i disable ADMIN ACCOUNT in vista x86

i have looked t the threads started by the admin but still no joy
for some reason i needed to be logged in as ADMIN to update the BIOS
so i ran the CMD *net user administrator /active:yes

now i have done what i wanted i now want to disable the ADMIN accoun
but running the CMD above but changing yes to *no* does not wor
i get an error 5 message
please can someone help me on this as i did this on my dads compute
and he wants it off :cry:
Thank You All so much In Advance*

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biddy79uk