My guess is that you are hit by "viral SSID" issue with Win XP SP2. Google search for the term to learn more about this.
The issue is that a client is setup in a wrong way, so the Win XP SP2 is setup to broadcast your SSID as ad-hoc. If a client near this computer sees it (and not your network) it will try to connect to the ad-hoc computer.
The only solution I found was to use GPO to push the profile to WZC and disable the clients possibilities to connect to ad-hoc network.
I never tried to look into the WLC for a status on hit clients, but I guess it could look like the one you are seeing (because of the similar SSID ??).