If you are sure the Etrex puts out a GPS signal on its interface (I think it does), you need a serial to USB adapter. It comes with a software driver to provide a COM port S&T can recognize.
However, with the prices of good USB GPS (such as the GlobalSat BU-353) being so reasonable, it would be easier and cheaper to buy one. Then you could leave it in your vehicle permanently and use the Etrex for what it does best. Also the USB GPS receivers are more sensitive than your Etrex.
Terry