A little strange, but there can be some latency in propagating NetBIOS information. Also check the properties of the wireless care and make sure the TCP/IP over netbios is enabled under the WINS tab, in the advance properties of the wireless card.
Also with the wireless enabled and the wired connection disabled. See what you can ping. Determine the actual IP address of the XP machine and the computer name. Open a command prompt and make sure you can ping both.
If successful, attempt to access a share on the XP machine using UNC but start with the IP address. Like;
Link Removed (substituting the actual address and folder name)
Keep us posted.