It seems that the wireless adapter on XP does not support WPA2 Security. Please enable WPA-PSK on the router and check if you are able to connect to the wireless network on the XP computer.
ENABLE WPA SECURITY
1. Open the browser (Internet Explorer, Mozilla Firefox or Safari) on a computer connected to the router.
2. Access the router, usually with http://192.168.1.1
3. Login to the router with the Username as "admin" and Password as "password" (please note that the Username and Password are case sensitive).
4. Click on Wireless Settings (under Setup) from the left menu.
5. Select security options as WPA-PSK. Enter a Password in the passphrase box. Apply the settings.
Configure XP computer:
1. Right click on My Computer icon and go to Manage.
2. Go to Services then go to Service and Applications.
3. Double click on Wireless Zero Configuration.
4. Change the Start Up Type to Automatic and click on Apply.
5. Click on Start and press OK to close the window.
6. Go to 'Control Panel', Click on Classic View in the left hand side menu and then open 'Network Connections'.
7. Right-click on the wireless Network Connection icon and select 'Properties'.
8. Go to the 'Wireless Networks' tab.
9. Check the option 'Use Windows to Configure this Wireless Connection' and click 'OK'.
Now please perform the following instructions to connect to the wireless network:
1. Go to 'Control Panel', Click on Classic View in the left hand side menu and then open 'Network Connections'.
2. Right-click on the wireless Network Connection icon and select 'Properties'.
3. Click on the 'wireless network' tab.
4. Under Preferred networks if you see any name in the list then highlight the name and click remove. Click OK.
5. Right-click on the wireless Network Connection icon and select 'View Available Networks'.
6. Look for the SSID (aka Network Name) of the Wireless router or access point.
4. Highlight thee SSID from the list that you want to connect to.
5. Type the security Key here and click on the 'Connect' button.
6. You should have successfully established a wireless network connection.