You need a System Repair Disk
to boot into to choose Recover Using A system image, which will detect the Windows Image Backup in the root of the external and reimage the HD.
You should have been given the chance to make the disk when you backed up the image if you used Windows 7 backup imaging? You'll need to find a Windows 7 computer of your bit version to make one,
or find a Windows 7 installation DVD to use it's Repair console on second screen after booting. You can borrow the DVD from a friend or download a safe "Official Windows 7 ISO download" from My Digital Life via a google search.