If you’re getting such error on solution/project load:
Project is targeting .NETFramework,Version=v4.0 which is not installed on this machine. You must download this framework in order to open and build this project. In order to proceed, you must select an option below.
Then probably you’ve uninstalled Microsoft .NET Framework 4 Multi-Targeting Pack.
Second – if you will try to reinstall it manually, you will get an error. The same MSDN article explaijs:
call NetFx_DTP.msi EXTUI=1 /log "%TEMP%\dd_netfx_dtp.txt"