The first thing to do is to fully erase (with a program like KILLDISK or DBAN) the hard disk of the PC/laptop.
Then the easiest way is to install the full (retail) version of Windows 7.
You can try to install an earlier version of Windows (98), then upgrade that to windows (XP), upgrade that to Vista, then upgrade that to Windows 7. Then you would need a full install file of the first OS, and upgrade versions of the next.
But since it's unlikely you can install Windows 98 to this machine (too fast a CPU, too large a disk, motherboard not compatible) I think that will fail.
Why did you buy a Windows 8 PC if - for some undisclosed reason - you need a Windows 7 PC? Seemingly, installing one of the available start menu's to make Windows 8 look like Windows 7 aren't sufficient for you. That's by far the easiest (install one program) and cheapest (free or 5 dollar, depending on what you choose) option.