If the computer is old and slow, don't use USB for networking - USB puts load onto the CPU, whereas an ethernet card doesnt. Also, ethernet is built for networking, so generally works straight out of the box without any issues (and since most cars use the same few chipets, most versions of Windows don't even need you to install drivers etc). Put simply, USB isnt built for networking, it is a jack of all trades and master of none.
Also, if it has USB 1.0 or 1.1, it will suffer a severe bottleneck as the capacity simply isnt there for modern broadband connections (its probably okay upto about 2Mbps though).
If you can, look for an Intel ethernet controller on the card, they are the mustard. I got an Intel Pro card on the bay for a few quid some years back, they were about £50 to buy at the time. But generally any chipset will do.