Please post back a screenshot of your maximized Disk management drive map with listings showing all columns so we can confirm the following steps based on what you presented so far.
Because C is Logical it cannot be marked Active to receive the System boot files from D so that it will boot on it's own.
You'll need to boot free Partition Wizard
bootable CD, rightclick C to Modify>Convert to Primary, OK.
Then again rightclick C to Modify>Set to Active, OK.
Now click on the HD to highlight it, from Disk tab select Rebuild MBR, OK, Apply all steps.
This may preclude the need to run Startup Repair - Run 3 Separate Times
to write the System boot files to C. If not boot the Windows 7 DVD Repair console or Repair CD to run Startup Repair up to 3 separate times with reboots. System Repair Disc - Create
You can now boot Win8 installer to install to D: select Custom Install, use Drive options to format D first, Win8 installer should configure a Dual Boot with Windows 7.
Another method is to boot the Win8 installer, format and install to D, then afterwards install EasyBCD to add Windows 7 to a Dual Boot Menu. Earlier versions of Easy are still free: EasyBCD 2.0.2 - NeoSmart Technologies
If you want a cleaner Dual Boot then install Windows 7 to another HD with all other HD's unplugged. After install plug back in other HD's, set preferred OS first HD to boot in BIOS setup, use one-time BIOS Boot Menu key to trigger other OS HD if needed. This keeps the OS HD's independent to come and go as you please whereas a Windows Dual Boot Menu requires surgery to remove one of the OS"s.
If you chose that last option you'll still need to recover the System boot files into C while deleting D to make Windows 7 32 bit boot on its own.