XP Pro does have issues with secure WiFi as it didn't support WPA2. Maybe you didn't reveal it was updated but I can only use what detail you supply.
Next up is WEP which this is very common. Fix? Move to WPA2 on the router.
There have discussions here before on this topic, but I can't find anything that relates to my problem. All of those answers addressed home systems not communicating. When in restaurants with free wifi, I get the familiar message that I'm connected to the access point but the internet can't be found. I have no problem at home, but can't get any internet on the unencrypted public systems. It has worked for me in the past, but not anymore, and I've tested it in several places which probably involved various routers. I'm using a Linksys WPC54GS on a Sony Vaio with XP Pro. Any suggestions that might be the cause of this? Thanks for any help.