Hopefully this will help others.

I have been 3 weeks on the problem of suddenly not being able to sign in to Live Messenger (with Plus) for 3 weeks, from my quick launch or toolbar.        MSN tried to help me by way of about 8 e-mails.   I spent the best part of 3 weeks searching the internet for 'new' clues because I had exhausted everything else.   Deleted stuff in the registry as per instructions, re-set my firewall half a dozen times.  Did msnmg.txt tests to send to MSN support.   In the end they decided it was most definitely my router.   I contacted my server and was told to temporarily disabled my router's firewall.  Nope !   Nothing.      Well, you name it and I had done it.    I also downloaded L.M 8.1 and 7.5 at least 8 times (uninstalled and reinstalled).           Two hours ago I stumbled on somebody's advice to at least sign in to Webmessenger for the time being.   I did this and signed in.   Then Webmessenger asked if I would like emoticons so I thought why not .......and it took me through to Live Messenger (oh not not again .....yawn!!)......but instead of downloading I decided to 'launch' it and I clicked on 'repair' L.M.    I watched as it took out old files and put in new ones.  I restarted and YAY IT WORKED !!!!!!!   

Really hope this helps somebody else with this problem.