Starting a couple of days ago I was suddenly unable to sign into Windows Live Messenger. The signin graphic would just spin endlessly, or I'd get a "service not available" message.
In case you have also run into this (many have), Microsoft did a bad thing (supposedly now fixed), and the effect is that bad data may have been cached on your PC, which prevents you from being able to log in.
Here's some info on the problem.
The solution if you are using Windows Live Messenger 8.0.x is to delete the registry key HKCU\Software\Microsoft\MSNMessenger\Policies. As always, don't attempt this if you are not comfortable editing the Windows registry.
Another related issue they describe is specific to ZoneAlarm firewall users, and that one is pending a fix as of this writing. They suggest dropping back to an older version of Messenger until that fix is available, so hopefully the registry key will fix it for you.