After I installed Live OneCare 1.5 beta on my computer didnt messenger want to start anylonger. The solution for this was to uninstall Messenger Plus!
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Its some kind of conflict between OneCare and Plus! since a friend of me got the same problem after he installed OneCare and he is also using Vista. I dont know any other antivirus systems to use on vista eighter.
Windows Live Messenger 8.0.0812.00 and MsgPlusLive-410.
OS: Windows Vista Build 6000