Try disconnecting your computer from the Internet before running iTunes.Again doing this once may sometimes help. Try holding ctrl+shift as you launch iTunes so it opens in safe-mode.Some users were finding that with the 12.6.1.25 build, for example, they had to use this method every time and modified the shortcut accordingly.
Doing this once may fix certain launch issues. If iTunes is apparently installed correctly try the following in turn until iTunes opens/closes/reopens/connects to the store normally:
