Open your saved email receipt from Microsoft then you can access your account online. Once logged in you can download 8 again, after it gets the files ready you can select 'install by creating media'. That will give you the iso file. You could use that to try a repair.
rdave13 - Thanks but this was my first Win8 install for a friend of mine at the pub. Didn't register it with Microsoft and turned off any usage feedback or anonymous stats when I installed it. (That might have been a mistake!)