Are you using the account named administrator? Or are you using another account, and that account have administrator rights?
A difference I noticed in Vista was that even if my user was in the administrators group, it didn't have enough privileges to install chipset drivers for example. So I deleted it and I'm only using the Administrator account itself, which has no restrictions like it was in XP.
In Win XP any account under the administrators group is at the top of the food chain. In Vista looks like the administrator user only has that right.
Try entering with the user named "Administrator" itself. The account may be disabled.
As usual, sorry for my english! (I think I'll make this my sig...)