OK. With Vista it appears that the answer is it all depends on various things including the version you have. I have Home Premium but I think you may have ultimate which has different features for UAC.
When first installing Vista an account is set up which has administrator rights. It is possible to add Admin accounts in the create new user account section. If you have upgraded the previous passwords should have been carried over if you set any. Perhaps try setting up a new Admin account first to see if your problem goes away.
To confuse things further the "real" Administrator account is disabled by default. An exception is if you upgrade from XP and like me the Administrator was the only active Administrator account, in that situation it remains enabled.
Vista, for security reasons does not like you using the hidden Admin account but access is possible. If you are desperate to do so type cmd in the search box of the start area, press Ctrl+Shift+Enter, agree the next UAC prompt and enter the following command
net user administrator /active:yes
When you have finished do the same again but swap yes for no. Note the space after administrator.
This is all new ground to me but the advice I have given is taken direct from a MS publication.