It's simply not possible, sorry. If you don't want to use the built-in backup, you'll have to resort to a third-party solution, I'm afraid.
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Microsoft has removed RSM (Removable Storage Manager) from 2008 R2 and Windows 7 in favor of new archiving technology. NTBACKUP which relies on RSM, will not be available in Windows Server 2008 R2 and Windows 7, and as such customers find it impossible to restore files or folders or entire volumes from old archives created with NTBACKUP on Windows 2008 Server and prior versions."
Windows Server 2008 R2 and Windows 7 cannot restore NTBACKUP volumes created by prior versions of Windows
http://support.microsoft.com/kb/2002414