Actually - by default on XP, there's also an account named 'Administrator' - it's just hidden from the logon screen unless you boot up in safe mode.
The account you create for yourself during installation is /also/ given Admin priveledges though, yes.
You can always try creating another user with admin privelidges and trying the same - or seeing if you can access the properties with the other account. I haven't seen it manifest like this, but it /could/ be possible you have a corrupt NTUSER file of some kind too. Hrm. (Just throwing up ideas... and babbling.)