What version of XP do you have? SP1? SP2?
The first release of Windows XP needs a WPA supplement to support WPA. If that is the case, you can download it from Microsoft.
You will find the update in this page:
Wireless update rollup package for Windows XP
http://support.microsoft.com/default...21120121120120
The update needs Sp1 to install.
If you have Sp2 you don't need the supplement and the problem lies elsewhere.
Regards.