As someone already said on this topic, everything you're asking is already possible and except if you reinstall your Windows every week, forgettingto change the log directory once shouldn't be that much of a problem
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One of the reasons this is not done by default is that around 75% of my users use Windows XP and this operating system lets you configure different user accounts for every member of your family When this is done, the "my documents" directory is different for every user so chatlogs are saved in different directories. Many people like to use several Messenger accounts and they seem happy to see that all their logs are saved in the same place by default.