Some additional information. Win10 pro X64. Rio without Update 1. I am not installing into the default folders. I am using C:\DelphiComponents as the destination for the folders.
The login I use is not an administrator - the installer is elevated at the time of being run. I did try a second time using Run As, but this made no difference.
Manual installation has enabled me to continue working.
MSBuild is not recognized as an internal or external command points to a corrupt installation. Typically this is being solved by re-installing MSBuild and RAD Studio.