All the tech's absolutely hate the usb/enet adaptors that we supply with set top box connections.
A network card is allways the best way to go, if the pc did not come with one fitted, then it is likely to be quite far out of warranty, as new pc's have had them fitted as standard for the last couple of years.
when it comes to networking, Usb is an acceptable stopgap solution, but a pci card (desktop) or pcmcia card (laptop) (wired or wireless) is the choice of anyone who knows the difference between the thing that they sit on & the bend in the middle of their arm.