I believe Vista starts out by assigning you (or the owner) as a system user status instead of Administrator. This is a good practice as it doesn't allow for immediate access in an intruder should enter your system. As Administrator one can do anything from downloads, installs, changing registry settings, delete files / folders, etc., basically full control.
As a system user, similar to a Guest account in XP, one is limited to what one can do within the system.
Vista allows one, by right clicking on a file, to set it to run as Administrator, thus giving full control.
Hope this helps a bit.