while Outlook is fairly easy to recover if you have access to the hard drive (search for *.PST files), it's a bit trickier with Windows Mail (tricky because certain information such as account settings, message rules, etc. are stored in the Windows registry).
To backup and restore Windows Mail messages, make a copy of all the files in the directory that contains the message and database files. The default location is
The contacts are stored in
In addition to backing up those files, export these 3 registry keys:
This is the most important registry key and contains nearly all the settings and rules for Windows Mail.
This key is kept for legacy purposes, but no longer contains much information. The Identities feature of Outlook Express has been removed in Windows Mail.
This key contains some information on the default accounts. In Outlook Express this key held all of the account information. In Windows Mail, most of the account information is now in .oeaccount files in the message store. See New Handling of Account Data on the Microsoft website. Note that when restoring a backed up copy, if Windows Vista has been reinstalled, the account passwords will need to be re-entered by the user manually.
Since the machine doesn't boot anymore, i recommend a repair installation first to backup these registry keys, then you may wipe the drive and go for a clean installation.