before adding the Wireless Adapter without having to enter a username and password? Are you a "member" of those sites and did you delete some cookies?
How is the ROUTER you are making a connection through set up for wireless security? If it is just limiting by MAC address you won't get a request for username and password, you will just be allowed or not allowed. If it is secured with WEP (Wired Equivalent Privacy) or WPA (Wi-Fi Protected Access) whoever set up the router would have privided you with the necessary password/passphrase.
I have a Dell Dimension 4550 computer. I upgraded with a Linksys wireless G pci adapter. Now when I go to certain sites a login and password screen pops up. I don't know what my user name and password is because when I added the hardware I didn't go through any password set up or anything like that. But it does say you can override web filtering, how can I do that?