Pull the hard drive and put that into some USB case for use later.
Slide in a new blank HDD and install the OS you want then when it's working plug in your old drive and copy out your files.
I had a 64bit Win 7 but I had restore problems so now instead of booting up,normally like before I now have to wait till a black page scrolls Win 32 operating system instead of what I had originally Win 7 64. I have restored that did not change anything . How can I preserve all my files but get back that 64 OS?