Unplug all other HD's and peripherals, Boot into BIOS setup to see if there is an EFI Boot Disk in BIOS Boot order. If so remove it, set DVD drive first to boot. If EFI BIOS also set to Legacy BIOS. Then set SATA controller to AHCI. Save changes and Exit.
Then boot into installer DVD or flash stick burned or written using tool and latest ISO here. How to Boot your Computer from a Bootable CD or DVD
At first installer screen Press Shift + F10 to open a Command Box, type:
SEL DIS 0 (after confirming Windows 7 target HD #)
CREATE PARTITION PRIMARY SIZE=102400 (for 100gb partition, adjust as desired)
FORMAT FS=NTFS LABEL="WINDOWS 7"
Next click Install Now to Clean Install Windows 7
to the partition you created. If it fails then report back at which step and the verbatim error message.
In addition read over these steps to understand the tools and methods which work best to get and maintain a perfect Clean Reinstall - Factory OEM Windows 7.
The steps are the same for retail.