Windows 8 issue

Hi, I am trying to install windows 7 on my laptop. I had windows 8 64bit installed. I was not aware of the GPT partition until a ran into this problem. The problem. I had all my files/documents etc on the d:/ I formatted the c:/ thinking I could just install windows 7 onto the c: but ran into error "Windows cannot be installed to this disk. The Selected disk is of the GPT Partition...