What software are you using to control the USB WiFi dongle? The standard inbuilt XP wifi manager or the software that came with the USB dongle. It's generally recommended on this forum to use the XP wifi Manager. What you can't do is have them both running at the same time.
Have you tried an alternate USB port?
Is the USB dongle set to 802.11b only on the problem computer (it's not necessarily the same on both computers).
The XP firewall is accessed via your network connections. Hightlight your WiFi connection, right click, chose Propeties and then Advanced to look at your firwall setting for that adaptor.