Nigel
To resolve your problem of corrupted files download a copy of Windows 7 Professional SP1 64bit and place it on a partition other than C on an internal drive. You can then carry out an In Place Upgrade using that copy of Windows 7 to repair and replace the corrupted files. At the end of this procedure you will need to reactivate Windows using your product key.
The procedure is described here:
http://www.sevenforums.com/tutorials/3413-repair-install.html
You are interested in the Step 4 option.
This is the original US language download:
https://drcdn.blob.core.windows.net/win/X17-24395.iso
There is a media refresh update:
https://drcdn.blob.core.windows.net/win/X17-59186.iso
You can read more about the available downloads here:
http://www.heidoc.net/joomla/technology-science/microsoft/14-windows-7-direct-download-links