Looking at Microsoft it seems I can do this without the key:
You can reinstall Windows 7 using Recovery in Control Panel, under Advanced recovery methods. This method reinstalls Windows 7, either from a recovery image provided by your computer manufacturer, or from your original Windows 7 installation files. You need to reinstall all of the programs that you added, and restore all of your files from a backup. For more information, see Choosing an advanced recovery method.
Will this be sufficient or is it a full reformat that I need?