Windows XP not identifying me as administrator

I tried installing Norton Antivirus, only to have the program tell me it
couldn't be installed because it didn't have access to the administration
file. I checked to make sure I am logged on as an administrator, and I also
emailed the Norton technicians who told me to try numerous things, even
changing the registry (which my computer would not let me do). I went ahead
and then bought McAfee internetsecurity suite, hoping it was just a problem
with the software, but I ran into the same problem. For some reason, the
computer will not identify me as the administrator, even though I have
everything checked for it to be that way. Can someone please help me?