I'm thinking of reformatting my hard drive and re-installing Win7.
Is there anything I need to do first, such as backup any particular file and put it back once I've reinstalled everything?
I've purchased a dozen books or so and they appear on my Cooler Ebook reader, but would assume there must be some file on the computer that I've got to keep?
Ensure that you have authorized with an AdobeID ( in ADE select Authorize Computer from the Library menu).
As long as you have done that then backing up your "My Digital Editions" folder will be sufficient, except for library books, which you will want to return as they will no longer be usable, after a format and reinstall.