Save an XP backup image using an app like Macrium - Image your system
so you have a path back.
Unplug all other HD's and peripherals,
Boot into BIOS setup, reset to defaults, save changes. (If an EFI BIOS you'll need to decide if you want UEFI BIOS vs. Legacy BIOS:
if UEFI (Unified Extensible Firmware Interface) - Install Windows 7 with,
or enable Legacy BIOS or Compatibility Support Module to proceed with normal install.) Set DVD drive first to boot, HD second, set SATA controller to AHCI, Save changes and Exit.
Then boot into Windows 7 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, or enter System Recovery Options
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, then Custom install 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.
After install you can adjust partition size or Create new partitions for data, etc. in Disk Mgmt.You are creating the partition here for the purpose of marking it Active which overcomes certain install failures.