I have not had a problem installing with UAC on, or off...
It's when I change state after installation that causes the problems. Some
programs, when installed, seem to remember UAC settings and information from
the installation process, then cause trouble if UAC doesn't match
installation settings when run.
Example:
Turn off UAC.
Install Word with UAC turned off.
Run Word.
Now, turn UAC on, then reboot.
Now, try Word again.
Same with Flash Player, Real Player, some AVs, etc. It's problematic if
you are going back and forth.
Best solution:
Choose whether you want it on or not and leave it.