If it only happens when you log in, then that is quite normal and not a bug at all.... When you log in Messenger does an extreme lot of stuff, of course your CPU usage is going to be higher then. And if it is the top most application running at that moment it automatically gets all the available CPU usage.
When you start up any other program (eg: Word or whatever) your CPU usage will go up too. That is how Windows works (and thank god for that, otherwise programs would randomly slow down etc)...
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