Correct me if I'm wrong but AFAIK you should at least run WinNT 4.0 SP6a to remove BUILTIN\Administrators safely.
I've tried this with SQL 7 on NT 4 SP6. Jobs kept failing, no matter how I've configured them. Finally I've found a knowledge base article which explained this:
http://support.microsoft.com/default.aspx?scid=kb;en-us;322988
Cheers,
Frank