If you go to your Documents folder, there's a folder in there called "My Weblog Posts", just copy that folder to your new PC and once Live Writer is installed, it will pick up your existing posts.
However, if you wish to transfer settings and plugins too, I would recommend my Backup utility which can be found at http://wlwbackup.codeplex.com. (It also does posts from your local machine).
SL - Windows Live Platform MVP
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