UAC's true purpose is to achieve the principle of least privilege and it does this by only elevating accounts to admin level when the correct credentials are given.
If your account is a standard user or power user and your try to do something that needs admin rights then you have to elevate your credentials using uac. In a sense it is really no different then doing a "run as admin command" but uac is automatic and application aware.
If your account is a admin group account then you don't have to enter credential you just have to say Ok when the uac prompt comes up. The only account in vista and win 7 that is not ran through uac is the built in administrator account which is hidden and disabled by default and many network adims on a domain enviroment delete this account for this reason but if active directory ever screws up and you computer/domain credentials don't line up you will find your self reimaging a machine and adding back to active directory.
Best to have the administrator account enabled but with a very strong password like 16 characters or better and lock it away and never use it. For our line of work make yourslef a admin group account and disable UAC to reduce headaches.
We use this as a uac replacemnt in most enviroments
http://www.faronics.com/enterprise/anti-executable/