Windows Vista comes with the new "Windows Mail", which replaces Outlook Express (that comes bundled for free with Windows XP). Many people end up using this email client because it's free and is robust enough for most people's email needs, and who doesn't use email these days?
Windows Mail stores your email, folders, accounts, newsgroup and other information in its own storage folder, which is hidden from the user by default for their own protection. What do you do if you want to change the storage location? Worse yet, what if you have a system problem and need to recover your email archive?