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If all you want to backup is your personal data and files and even the settings, then it does suffice to only copy the user folder in Macintosh HD / Users / YOU (Mac OS X) and the user folder in C:/Users/YOU (Windows Vista + 7) or in C:/Documents and Settings/YOU (Windows 2000 + XP).
Or you can manually sort through them and only copy what you specifically want, if copying those home directories is to general for you.

You may leave out the Library folder in Mac OS X, as it only contains preferences and support data, but it also contains bookmarks and your mail, if you use a mail client.
 
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