Steve Dower added the comment:
Glad you got it sorted out. I've done exactly the same thing too.
The 3.5 installer is designed to have no impact on earlier versions, and also
has different default directories for 32-bit vs. 64-bit, which will help
prevent these problems in the future.
New submission from Ryan Neve:
Much like issue 22648. Windows 8.1, I had version 3.4.1 and installed 3.4.3
without first uninstalling earlier version. Tried uninstalling older versions
and it failed with:
A program required for this install to complete could not be run.
Windows uninstall
Ryan Neve added the comment:
Turns out I had accidentally installed 3.4.3 32 bit over 3.4.1 64 bit. I
re-installed 3.4.1 32 bit, then un-installed it, then I was able to install
3.4.1 x64.
I also tried installing and un-installing 3.5 which worked fine but didn't fix
anything.
--
