Hello Holly,
I am no expect when it comes to UAC. I do know that it is place to help ensure that programs have to prompt for access to the system and that IE usually isn't one of the programs that would display the UAC prompt. I would be concerned with a potential virus or other malicious software trying to get access to your system.
There is a post on the Vista forums that you may find helpful, before moving forward:
You need to identify what program/activex control or other add-on is causing
the UACmessage to appear. This could be a malicious program that could harm
your computer.
What that message is trying to tell you is that "A program is trying to get
complete and unrestricted access to your entire computer".
First, determine what steps you are taking that makes that UAC prompt
appear. Next, go to Tools/Manage add-ons/Enable or disable add-ons.
Near the top of that dialog is a drop down menu that gives you different
views of the add-ons. Using these different views, disable a block of the
add-ons and then recreate the steps that make the UAC message appear. If the
message does not appear, re-enable each of the add-ons in that block,
one-at-a-time, checking to see if the message appears after enabling each
one. When the message appears, you have identified the culprit.
You can then go to the website of the maker of that add-on and make sure it
is legitimate. If it turns out to be OK, check that website for an updated
version of the add-on for Vista/IE7.