you need if it a hp or pre built the restore cd or software. if it one that you build you need the windows 7 cd.
boot from the restore or windows 7 cd. go into advance mode and delete all the partion on your main hard drive. then reboot and let windows 7 make two new partion the system restore and c partion. then it install windows 7 on a clean drive.
when windows 7 boots you just need to install the drivers for your video card..sound..network card and mb if windows 7 did not. when you wipe windows 8 from your pc back up any data you want to keep.