This is how to remove cached user credentials that are used for PEAP authentication in Windows XP, after doing the steps below, Repair the connection and it will ask for credentials again.

http://support.microsoft.com/kb/823731

To remove cached user credentials that are used for PEAP authentication

Important This section, method, or task contains steps that tell you how to modify the registry. However, serious problems might occur if you modify the registry incorrectly. Therefore, make sure that you follow these steps carefully. For added protection, back up the registry before you modify it. Then, you can restore the registry if a problem occurs. For more information about how to back up and restore the registry, click the following article number to view the article in the Microsoft Knowledge Base:

322756  (http://support.microsoft.com/kb/322756/ ) How to back up and restore the registry in Windows

To remove your cached user credentials that are used for PEAP authentication, delete the HKEY_CURRENT_USER\Software\Microsoft\Eapol\UserEapInfo registry key. To do this, follow these steps:

1. Click Start, and then click Run.
2. In the Open box, type regedit, and then click OK.
3. Locate, and then click the following registry key:

HKEY_CURRENT_USER\Software\Microsoft\Eapol\UserEapInfo

4. On the Edit menu, click Delete.
5. Click Yes when you are prompted to confirm the deletion.
6. Exit Registry Editor.