If you are encountering the error 80048821 when you sign into Messenger try the following:
1. Visit http://account.live.com
2. Click Sign in with a different account.
3. Enter the Windows Live ID that you use with Messenger and the correct password.
4. Click Sign in to verify that you can successfully sign in by using this account.
5. If you are unsuccessful, try to reset the password. To do this, follow these steps:
5.a. Visit http://account.live.com
5.b. Click Sign in with a different account.
5.c. Enter the ID, and then click Forgot your password.
5.d. Follow the steps in the wizard to reset your password.
5.e. If you need help resetting your password because you cannot remember your secret question and answer or alternate e-mail address, contact Windows Live ID support. To do this, visit the following Web site, and then select Windows Live ID: http://support.live.com
5.f. After you successfully reset your password, go to the next step to update the settings.
6. If you are successfully signed in by using your account, remove the saved credentials, and then reenter the updated credentials. To do this, follow these steps:
6.a. In the Messenger sign-in window, select your e-mail address from the E-mail address drop-down list. If you do not see your ID on the list, go to step c.
6.b. Click Forget Me, and then click Yes to confirm the deletion.
6.c. Type your e-mail address and your password.
6.d. Select the Remember Me and Remember my password check boxes to save your sign-in information, and then click Sign in.