I've just walked through nearly the exact experience of the original poster, except my issues were with the latest mandatory update to 6.4.2.541639 (64 bit default Windows release).
Before coming to this thread, I tried uninstall reinstall, no luck
Then I tried another uninstall, and now I can't even uninstall. And further, the contents of the folder C:\Program Files\SecondLifeViewer are locked. Trying to delete or rename the folder gets me an error telling me the contents are in use, even when SL is not running.
I had to create a new installation folder and download/install the RC version 6.4.1.541204 (64bit).
I did nothing to disable my AV (Windows Defender) or Malwarebytes, but the RC installation went off without a hitch.
In summary, I found a workaround, but it's just that, a workaround. I hope I won't have to create a new installation for every update going forward.