Win 7 won't disappear

I installed win 8 on my win 7 because I thought I would still use the win 7. Turned out I didn't (and the win 7 never worked again) but now I can't make the selection screen that appears on start up go away. I have Installed my win 8 again 2 times and always chose the way that the old windows cannot be used. Still no success. What should I do? Format the whole C: first?

Cleanup the bootmgr. See Step3 of this tutorial: Dual Boot - Delete a OS - Windows 7 Help Forums

Clean the windows.old