Back up your files, unplug all other HD's and peripherals,
Reset the BIOS to defaults: Clear CMOS - 3 Ways to Clear the CMOS - Reset BIOS
. Then set SATA controller to AHCI, set DVD drive first to boot, HD second. Save changes and Exit.
Then boot into installer DVD or flash stick burned or written using tool and latest ISO for your licensed OS version here.
At first installer screen Press Shift + F10 to open a Command Box, type:
SELECT DISK 0 (after confirming Windows 7 target HD #)
CREATE PARTITION PRIMARY
FORMAT FS=NTFS LABEL="WINDOWS 7"
Next click Install Now to Clean Install Windows 7
to the partition you created.
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.