I have the same problem with Civil 3D 2018. I have tried *everything* - clean uninstall\re-install with a fresh setup.exe downloaded with AVA today, Reinstalled from Add/Remove Programs, ran Repair a couple times in a row (this worked for one person), uninstall/re-install .Net and C++, empty %temp%, disable anti-virus, manual cleanup of residual files and registry entries, run MicrosoftProgram_Install_and_Uninstall tool several times, *then* tried to install, bla bla bla - believe me, if it's out there on the web I have tried it.
The only ultimate solution I have found in researching this is to reinstall the OS. Unfortunately, this is really not a realistic option for me unless I want to send my machine downtown to IT for a couple weeks, then re-install a complete suite of office and surveying & engineering production packages.
I'm relieved that the .NET/C++ uninstall/reinstall didn't break the other apps I have on this machine. This seems to be tenuous and possibly dangerous advice.
It would be a good thing if someones found a decent solution to this problem.