Arduino Nano not recognized.

I have a problem with my arduino Nano.

I have build a RGB LED strip controller with a arduino Nano which is build into a PC housing.

But when I boot the PC the arduino is not recognized and displayed as a unknown device.

When I boot the device with the arduino disconnected and connect it when windows is booted he works as it should.

I just need to have a functional FTDI virtual COM port.

Any idea what the problem could be?