>>There isn't a Proxy or firewall in the middle?
>
>No

Some things you could check (thanks to Jonathan Kay - Messenger MVP):

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 @ http://memberservicesnet.passport.com/memberservice.srf 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:
regsvr32 softpub.dll
then click OK

Hope any of this helps. If not, please let me know.

Best luck,