Windows will automatically default to the faster of the two - theoretically.
"In XP, the default interface metric for all network adapters is set to Automatic, which causes the adapter to use the fastest available connection. But for some reason, the wireless card installation program had set the wired interface metric to 1 and left the wireless interface set to Automatic. This change had the effect of setting the wireless metric to 0 (although you can't actually set the metric to 0, XP gives priority to an interface metric set to Automatic over an interface metric set to 1). The OS uses the lowest-numbered interface (in this case, the automatically set wireless interface) for all default network traffic. Resetting the interface metrics for all network adapters to Automatic returned the default behavior and gave priority to the fastest network."