Windows Live Messenger could not sign you in because the sign-in name you entered does not exist or your password is incorrect. Error code: 80048823
Before you start. Did you enter correct Username and Password?
1. The date on your computer needs to be set properly. Double click the clock verify that the time and date are set.
2. If your password information is not saved, verify that you are typing it in with the correct case (uppercase or lowercase).
3. Change your password at http://passport.com to something smaller (under 10-12 characters), then try signing in again.
4. If you use a firewall (like ZoneAlarm, Norton Internet Security etc.), it's possible that Messenger doesn't have the correct rights to access the Internet, especially since you upgraded. You may need to re/add Messenger to the allowed list of programs in your firewall if this is the case.
5. If you disabled your firewall in the past, it still may be blocking Messenger. Try restarting it and see if that helps the situation. You also might try uninstalling an installed firewall, to verify that it isn't causing a problem (even if it is disabled).
6. Clear your IE cache and cookies. Open Internet Explorer, click the Tools menu, then Internet Options, then click the Delete Files button, and when that's complete click the Delete Cookies button.
7. Check your IE Security settings. Open Internet Explorer, click the Tools menu, then Internet Options, then Advanced tab, scroll to the Security section, and verify that "Check for server certificate revocation" is unchecked. Also verify that 'Use SSL 2.0' and Use SSL 3.0' is checked, then click OK.
8. Click Start, then Run, and enter the following:
then click OK
and do the same for the following:
9. If using MSN Messenger 7, click the Tools menu, then Options, then Connection category, click the Advanced Settings button, uncheck all the options, click OK twice and then try again.
Update(4.2.2008): check helper's solutions posted below