okay, just to get it from the beginning.
In Vista only the built-in administrator account works as real administrator.
Every other user in the administrators group have the admin rights removed from their token, so they work as normal user (this is the so called UAC feature).
So, to do admin tasks like registering a dll you have the following choices:
-run the cmd elevated
(start-all programs-accessories-right click "command prompt" and choose "run as administrator, you can also set in the "command prompt" properties that you want to run it always as admin
or
click the start orb, type cmd in the search bar, and press ctrl+shift+enter)
-use the builtin administrator account
-disable UAC alltogether (control panel-user accounts-Turn User Account Control on or off)