Downgrading from Win8 to Win7 on a pre-built OEM Laptop/PC

[OilSheikh]

Using Rufus i prepared a usb drive with a MBR (not GPT) partition scheme for bios or UEFI computers in NTFS format.
Disable Safeboot in BIOS
Switch to Legacy from UEFI option, if available

At first installer screen Press Shift + F10 to open a Command Box, type:

DISKPART
LIST DISK
SEL DIS 0 (after confirming Windows 7 target HD #)
CLEAN
CONVERT MBR
CREATE PARTITION PRIMARY SIZE=102400 (for 100gb partition, adjust as desired)
FORMAT FS=NTFS LABEL="WINDOWS 7"
ACTIVE
EXIT
EXIT

Next click Install Now to Clean Install Windows 7 to the partition you created.

[ Credit to sevenforums ]