I think you need to deactivate your IEEE 802.1x security in Windows XP. This is turned on by default in XP and does not hurt wired networking but does have a negative affect on wireless connections.
To turn it off:
Go to your Network Connections - numerous ways to get there, but it's in the Control Panel. Highlight your wireless connection and click on Properties. Click on the Authentication tab. Uncheck the "Enable IEEE 802.1x authentication for this network" box.
Good luck! Let us know if this fixes your problem.