Cannot install itunes on win 8.1

Question

I have a Win 8.1 Pro HP ENVY 15t-k000 x64-based Intel Core I7-4710HQ with 16GB Memory, 100
GB SSD and 1TB HDD.  When I originally got the system I downloaded and installed iTunes (don’t  remember which version it was) on it with no problems.  Eventually an iTunes update asked to be applied (don't remember which one), it did not go on smoothly but
iTunes has not worked since.  If I try to start iTunes would get message "The program can't start because
MediaToolbox.dll is missing.  Try reinstalling the program to fix the problem.  ITunes
was not installed correctly.  Please reinstall iTunes.  Error 7 (Windows error 126)."
I downloaded iTunes64setup.exe (Ver. 11.3.1.2 on 8/17/14) and it would not install for
various reasons.  I got back to the problem on 3/2/15 and downloaded iTunes6464setup.exe ((Ver. 12.1.1.4) and it will not
install.  Primarily gives error "An error occurred during the installation of assembly 'Microsoft VC80.CRT,
type=win32 version "8.0.50727.6195",public Key Token='1fc8b3b9a1e18e3b', processor architecture"amd64".  Please refer to Help and Support for more information.  HRESULT: 0x80070002.
Now, I have  since removed all iTunes programs as per the removal list (three different
Registry cleaning and removal tools) to make sure I got everything and tried to
reinstall.  Got the same results (the VC80 error).   I ran a lot of various tests, even went back and tried to install the Ver. 11.3.1.2
download and didn't get the above VC80 message but got many more assorted
messages, mostly concerning missing .dll entry points, some of which I loaded
but to no avail.
I am back to wanting to work with the latest iTunes6464setup.exe (Ver. 12.1.1.4).  I believe VC80 message above has to do with
iTunes trying to install an update to a Visual C ++ 2005 Service Pack Redistributable but am not 100% sure.  I
have also tried several other methods of install e.g "Clean" system install, Safe Mode, all compatibility options, etc.  At this
point I am about to give up on iTunes as a media player. I have read all the internet solutions that I
could find but none of them seem to address my exact problem.  Let me remind you there is a distinct
difference between Windows 8.0 and 8.1.  A lot of Win 8.0 discussions just does not apply to Win8.1.
   Thanks for any help or insight you can provide.

Answer 1

For general advice see Troubleshooting issues with iTunes for Windows updates.
The steps in the second box are a guide to removing everything related to iTunes and then rebuilding it which is often a good starting point unless the symptoms indicate a more specific approach. Review the other boxes and the list of support documents further down the page in case one of them applies.
The further information area has direct links to the current and recent builds in case you have problems downloading, need to revert to an older version or want to try the iTunes for Windows (64-bit - for older video cards) release as a workaround for installation or performance issues, or compatibility with QuickTime or third party software.
Your library should be unaffected by these steps but there are also links to backup and recovery advice should it be needed.
There is a mild hint from another thread that disabling Windows Firewall during the reinstall might help with the Microsoft.VC80.CRT error. I'd also try the "for older video cards" version if you haven't already done so.
tt2