Ryan Zahra has provided you with a link to the COM documentation, and obviously you can use the fileformat as you intend to, but you may not want to do it this way.
You may find the
Virtual Hard Disk Image Format[
^] more suitable, and you may even be able to boot from your backup
http://en.wikipedia.org/wiki/VHD_(file_format)[
^] - if you are running Windows 7 Enterprise, Ultimate or Windows 2008 R2 Server.
The thing about backups is that when you need them, it's kind of nice to get them up and running without a lot of fuss - at least it's something worth researching a bit further.
You can even use Windows Disk disk management to create and mount VHD files, so you can get it done without writing a single line of code :) (Open Administrative Tools -> Computer Management - goto Storage -> Disk Management and right click on the node in the treeview, you'll see "Create VHD" and "Attach VHD")
Regards
Espen Harlinn