Ok, lets try and work out what's blocking you from signing in.
Carefully check Control Panel > Programs list and remove any Messenger add-ons (including Plus!), as well as
these problematic programs.
Next thing, what security do you have? Firewall? Internet security? Anti-virus? I'd recommend removing them and restarting your computer, it's the only way we can be sure.
You should also try leaving Messenger trying to connect (with the spinning 'buddies') and eventually it should give you an error message + code, which can help determine the problem.
Try to using the Connection Troubleshooter (you don’t have to login to Messenger to do this). You can find it in Messenger -> Tools -> Options -> Connection tab. What is the result?
Also on the same tab, click Advanced Settings, then all the fields should be blank. Try clicking 'Test' on TCP, it should say 'Success, you can connect to the Windows Live Messenger service.' if it works.
Source:
Windows Live Messenger Connection Problems etc.
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