You will need to remove the HDD and plug it in to a system that you do have access too. The second system will see it as a secondary drive and upon access will likely ask if you want to take ownership of the files and reapply permissions. Once you can access the files copy them to a safe place that you can access later. Then reformat and install Windows and return the files to where you want them. Of course don't forget your new user name and password.