Halo Problem

I just bought a copy of Halo and installed it with no problems. However,
whenever I try to actually play the game, a message appears saying 'Please
login with administrator privilages and try again'.

There is only one account on my PC and that is most definately an Admin
account. I have tried making another account with admin privilages but it
still does not work. As a related note, I also get this problem when I try to
use Windows Update (it just says that I can't use the site because I am not
an admin).

I think this might be to do with the last time I had to re-install WinXP.
The code that came with it had expired and I had to get a new one by ringing
up some number it gave me. In all the other installs I have had this problem
has not occured once.

Any help is gratefully appreciated.