As of recently, she was able to use this computer to connect to a neighbor's/friend's secured wireless netowork and another neighbor's/friend's unsecured wireless network.
What's really strange, now I noticed, that this problem is occurring with any Network in range on the list. Without actually looking at the system, it is hard to tell exactly the problem but here are 2 general things to explore: Did you by chance install any of the Comcast utilities on her machine?
By default, XP will re-authenticate with the user credential after the user logs-on (there is no way to do computer-only with XP, I believe).
When I try to connect using the windows zero configuration utility it gets stuck validating identity.
It says something about windows not finding a certificate.
I thought that at first, perhaps, a virus and/or malware somehow got by and changed/altered the wireless settings, though the antivirus software and Malwarebytes software is up-to-date and is showing no problems. One of these may be doing strange things with the connection. If that does not work it could be a newtork card driver issue. However, before you do that I would advise you to make sure that you have the driver disc for that netcard (or for the notebook if it is an internal Wi Fi netcard), because Windows will often not have built-in drivers for any given netcard.
One way to restore networking, especially wireless, is to go into control panel, system and delete the network card. Since you mentioned that this notebook had previously been setup for multiple wireless profiles, my suggestion would be to delete all existing wireless profiles and see if that allows you to then create a new profile for the Comcast gateway.
Also, make sure that you are using WPA(2) Personal (PSK) for both the Comcast gateway, and for the Wi Fi client that you are setting up; don't try to use "WPA-Enterprise" or "802.1X" security because that almost certainly will require resources that you don't have (resources such as a security certificate for that connection).