Solution to Windows XP wireless AES/WPA2 problem

If your Windows XP computer doesn't seem to recognize AES
(or WPA2) and/or it won't ask for network key ,make sure
that you're running Service Pack 2 and that you have
downloaded and installed KB 917021 or install SP3.

http://support.microsoft.com/kb/917021/en-us

Download the Wireless Client Update package here.