4. If you use a firewall then you may need to allow add Messenger in your firewall.
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 for signing.