I have windows 7 64 bit on several computers on the same iTunes account. Usually the iTunes updates install ok. When the latest one - v 126.96.36.199 was offered on my desktop it failed with a "Lost MSVCR80.dll" error. I tried and failed twice to install, and also failed with the manual method. I solved it by a system restore to the old version. On a laptop it installed ok. I have read of many problems on the internet with this version, particularly of installation issues an when installed of non recognition of devices. I wonder if this is a general problem with others? I tunes annoying suggestions to totally uninstall all I tunes related programmes first is very annoying.
I think this is a very widespread problem and there are many posts on it via Google. It has to do with the fact that when (for W7) .net framework 4.5.1 replaced former versions (as opposed to versions up to 3.5 kept previous versions) , some elements of Microsoft Visual C++ were lost. There seems little doubt that . for some, this update has been a disaster.
I solved it in the end by the recommended uninstalling of all Apple applications except Cloud, deleting all "Programme files" folders relating to Apple ten installing the new version. I believe there was a glitch to do with the fact that with .net framework 4.5.1 previous .net framework versions were deleted, and somehow this affected some PC's. A long winded an annoying episode.
He tried system restore and it didn't work. I visited him and went for John Bunyan's solution and deleted all Apple applications, then made sure the relevant Registry entries were deleted and downloaded and installed the latest iTunes version. It worked and I now have a very happy neighbour! He asked me to convey his thanks to you all.