If your PCs have real RS-232 serial ports (rare these days), you can use those directly. Otherwise, you will need a USB<->RS-232 serial adaptor cable.
Correction: I thought my Arduino did have an RS-232 port, but I was thinking of another board. If your board only has a TTL-level serial port (ie. direct from the ATMega328), then you need either an RS-232 level translator, or a USB<->TTL serial cable. A suitable cable is available from Adafruit (and no doubt, others).