Arduino Upgrading from 1.0.6 to 1.6, code doesn't work anymore
You should not have added libraries to the Arduino files in the "C:\Program Files (x86)\Arduino" folder in the first place.
That is only for the Arduino program files and standard libraries that are included with the Arduino IDE.

Could you uninstall the Arduino and serial drivers. Remove also the "C:\Program Files (x86)\Arduino" folder, to be sure that absolutely nothing is left behind, because that will interfere with a new version that will be installed. Remove also the hidden "arduino15" folder(s).
Do you know the location of your sketches, it is in the preferences and is often "Documents\Arduino". Go to that folder and go into the "libraries" folder. Remove unused libraries or libraries that have been included in the Arduino files.
Do you have Java installed ? Some Java versions are not compatible with the Java files of the Arduino. If you don't need Java, then uninstall it.

Install the newest Arduino 1.8.3 from Arduino.cc Software.
Use the Library Manager in the Arduino IDE, and search for modbus. Install the "ModbusMaster by Doc Walker" with the Library Manager. The modbus library that will be installed is this one: Github 4-20mA ModbusMaster.

There has been a few changes, for example with PROGMEM. All PROGMEM data must now be 'const'. You might have to make a few adjustments.