Error number: 0x800A0046

Hi,
No one seems to have this problem -- at leasty that I can find. Since I
updated my XP Pro to SP2 I have consistently gotten this error after pressing
the "Express Install (or Custom). On looking it up, I find that it involves
permission denied. (Priviledges) however, I am ALWAYS the adminsitrator
(signed in, etc) MS Support, after three months of trial and error suggested
I reinstall!!! Now, that is NOT a solution since all else works fine. I was
told to remove all "unauthorized users" from the group but since I have only
authorised users in each group, and noone will tell which ones are NOT, I'm
stuck -- HELP??
.