First make sure that your WinXP support WPA2 (see note 2).
If after applying this patck WPA+AES (WPA2) does not appear as an option, it means that your Wireless Card drivers do not support WPA+AES.
Windows does not make the decision about the encryption, it comes from the Wireless Device Drivers.
In general.
From the weakest to the strongest, Wireless security capacity is.
No Security
MAC______(Band Aid if nothing else is available).
WEP64____(Easy, to "Brake" by knowledgeable people).
WEP128___(A little Harder, but "Hackable" too).
WPA-PSK__(Very Hard to Brake ).
WPA-AES__(Not functionally Breakable)
WPA2____ (Not functionally Breakable).
Note 1: WPA-AES the the current entry level rendition of WPA2.
Note 2: If you use WinXP and did not updated it you would have to download the WPA2 patch from Microsoft.
http://support.microsoft.com/kb/893357
The documentation of your Wireless devices (Wireless Router, and Wireless Computer's Card) should state the type of security that is available with your Wireless hardware.
All devices MUST be set to the same security level using the same pass phrase.
Therefore the security must be set according what ever is the best possible of one of the Wireless devices.
I.e. even if most of your system might be capable to be configured to the max. with WPA2, but one device is only capable to be configured to max . of WEP, to whole system must be configured to WEP.
If you need better security and one device (like a Wireless card that can do WEP only) is holding better security for the whole Network, replace the device with a better one.
Setting Wireless Security -
http://www.ezlan.net/Wireless_Security.html
The Core differences between WEP, WPA, and WPA2 -
http://www.ezlan.net/wpa_wep.html