When running the installer 'SolarWinds-NCM-v7.5.1.exe' I keep getting a Loading.... Error opening file for writing C:\....\Temp\2\SWOrionSetup\Castle.Core.dll
there was probably a process running from NCM.. like a backup job or something that prevented the file from being overwritten. When you rebooted the process got killed and the 7.5.1 install worked.
I had to install 7.5.0 and reboot for it to extract/install 7.5.1