UAC is User Account Control. It is that function that brings up a box every time you try to install something and asks you if you really want to do that. It can cause issues with some installs under certain conditions.
One of the easy ways to turn it off (and on) is type msconfig in the start search box and press enter. When you see the System Configuration window, choose the Tools tab.Choose Disable UAC from the list, click the Launch button and restart the computer. Try your install again and see if that helps.
If you want to restart UAC, it is in the same place called Enable UAC.