This seems to work just in case you are testing various versions of Windows and have problems with Booting
You CAN use the repair option on a different level of Windows to the one you are currently using -- so long as both the install disk and the target Windows system are both x-86 or x-64.
I've repaired a Windows 7 Ultimate install with the repair option booting from a Windows 7 Home Premium disk and also the other way around
You don't need (for the repair function) to enter a serial number.
Note that you should only use this for BOOT problems and only if you don't have the correct disk immediately to hand --saves downloading a 4 GB or so ISO --even using a FAST connection it will take a while.