UAC (User Account Control) is the new security model for Vista. Basically, it tries to run everything with the least expected permissions, even if you are running as an admin. Based on file type, file name and other parameters, it tries to figure out what permissions the application is going to need. If it thinks the application will need extended permissions, it will ask to continue (if you are an admin) and attach administrative permissions to the application.
Daniel Moth has posted a collection of links that explains UAC to the point of saturation. It helped clarify some things for me.