You can get USB to RS-485 dongles for the PC that just look like a serial port.
This is half-duplex but you could make your own full-duplex version easily. The FTDI data sheets have examples but it's pretty simple.
And at the Arduino end you can get an RS-485 shield, once again I think half-duplex but that doesn't really matter and you could make your own full-duplex version for maybe $20.
Full- or half-duplex doesn't matter much, it just determines how you write your code.
Rob