the IoT devkit uses a different USB driver then other images: it offers USB mass storage & serial.
If you install the IoT devkit image and connect the board to your Windows machine by USB, Galileo will appear as an additional device. In the "windows-driver" folder you'd find the driver to install (device manager -> CDC device -> update driver -> ... -> Have disk).
After the installation the device should work fine for the Arduino IDE again.
I mean the windows driver for galileo 2 IoT devkit version you were talking about
