Back up your files and a System Image if you want a path back to the OS you have now, unplug all other HD's and peripherals,
Boot into BIOS setup to set DVD drive first to boot, HD second. 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 for your licensed 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 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.