When you connect to a network for the first time, you will be prompted for your user credentials. XP will save the credentials you supply and use them for all future connections to the network. You can clear out the credential cache by editing the registry. Fire up the registry editor, and delete the HKEY_CURRENT_USER\Software\Microsoft\EAPOL\UserEAPinfo tree.
You can also get tracing on the supplicant by setting HKEY_LOCAL_MACHINE\Software\Microsoft\Tracing\EAPOL to 1. There’s a key that will specify the location and size of the resulting trace file. I have generally found tracing on the authentication server to be more useful, though.