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!
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