The update supports the additional mandatory security features of the IEEE 802.11i standard that are not already included for products that support WPA. Additionally, after you install the update, Windows XP will display previously hidden Service Set Identifiers (SSIDs) in the Choose A Wireless Network dialog box. This functionality makes it easier for you to connect to public Wi-Fi networks to which you have not previously connected.
You can find the update HERE
You can read the Knowledgebase article here.