No idea really.
However, I would start trying random things and see if anything helped.
You could just reseat your WLAN card in the back. Doubt that's it though. Make sure the antenna connections are on tight.
You could open the device manager (right click my computer, hardware, manage hardware or whatever) and find the device driver for the WLAN card and UNINSTALL. It isn't really uninstalled, the driver is still there, its just not hooked into the OS anymore. Then reboot. It'll find the card and reinstall the driver again automatically. Might fix something.
You could use the help system to locate the wireless repair utility. That might find something.
Or Try the Microsoft PC Advisor, though I suspect that won't work on XP...