Asus X550C Windows 8.1 cannot login and cannot reset - HELP!

Firstly I should say that I am a self-employed (for 10 Years!) PC fixer but I am stumped at this one. I've also decided I hate Windows 8, UEFI and all that they bring with them ........

A customer of mine (who probably won't be a customer of mine much longer if I don't get this fixed!) recently bought a new Asus X550C with Windows 8 installed. She asked me to set it up for her and configure it. I configured the laptop and ensured that all App Updates were completed and all Windows Updates. I also installed the Windows 8.1 update.

Her old laptop was a Windows Vista Home Premium machine and I used Windows Easy Transfer to transfer her 'stuff' over. With hindsight this was a mistake!!

The laptop was working fine when I handed it over to her.

Two days after handing the new machine back I got a call from the customer advising me that she could no longer log in to the machine as her login name had mysteriously changed. The only login name now available was someone who had used her old laptop some years ago but who she was no longer in contact with it being the ex-boyfriend of a relative. Things had changed when she ran Skype for the first time and I can only assume that on her old machine there was a Skype profile for this mysterious man and some how with using WET the settings had been transferred over to the new machine and when she had tried using Skype somehow the Microsoft Account linked to the new machine got linked to this chaps Skype/Microsoft Account.

After realising we were locked out I tried to 'Reset the PC', so that I could set it up again. However the machine would not run the Reset routine as it reported that 'Unable to reset your PC. A required drive partition is missing.' No I definitely did not delete a partition. I can only assume that Windows 8.1 Upgrade did something to the partition.

I did try to use a Windows 8.1 OEM DVD to install Windows 7.1 from scratch but this wanted am installation key which is not available on the laptop. I understand from Asus support that there is no CoA on the machines but that the key is built in to the BIOS/Recovery Partition.

I have bought a set of recovery disks from Asus/MentorMedia and tried to use these to recover the machine. When I go into the option to 'Use a device - Use a USB drive, network connection or Windows recovery DVD' and select 'UEFI:MATADVD-RAM' I hear the DVD being accessed and get the prompt to 'Press any key to boot from the CD or DVD' and do just that but then the machine continues to boot into Windows from the hard drive.

So now I am stumped. The options we have are for me to RMA the laptop via Asus so that they can reimage the drive, or for the customer to return the laptop to the store and get them to deal with repair or replacement.

I'd prefer to sort this out myself if possible. Does anyone have any ideas how I can sort the problem out myself.