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Mostly we can not remove/delete/erase recognizable system partition from within the system environment using native disk management as it is protected.
You can do it through third party disk/partition program such as Partition wizard, Easeus, etc.
Or, using bootable utility disk.
Edit: When you done erasing, you need to remove Windows 8 from boot menu (if it was dual boot) using EasyBCD from W7 environment.
You cannot delete Windows 8 (drive D) as it is Windows System partition - the System partition in Windows has the boot files !
1. Mark Windows 7 partition as active.
2. Reboot to Windows 7 Repair CD/DVD/USB and run StartUp Repair up to 3 times with rebooting.
This is the recomended way on "sevenforums".
Partition - Mark as Active - Windows 7 Forums
Startup Repair - Run 3 Separate Times - Windows 7 Forums
Not sure if you've handled this yet, but here's an easy way how.
Copy bootmgr file from D: to C:
Copy boot folder from D: to C: and skip over the files it tells you it can't copy.
Open an elevated command prompt and run this command:
bcdedit /export C:\boot\bcd
Now set C: to be the active partition in Disk Management. Reboot and now you can format D:
Be sure to also remove the reference to D: in the bcd. You can use EasyBCD for that.
EasyBCD's "Edit Boot Menu" tab has a box to check to "Skip the boot menu".
Why not just clean-up the MBR with EasyBCD.
From what I recall, EasyBCD has an option of "migrating" the Windows boot loader files from one partition to another, and in the process, updates the MBR accordingly.
I think also have the option to Repair the boot files.
So, if the boot loader files are in the Win7 partition, after removing the Windows 8 partition, you would just use this EasyBCD Repair option and you would be OK again.
Fixed.....Just delete windows 8 CP via windows 7 installer......and fix booting with windows 7 repair tool on windows 7 DVD