Here is what you need to do:
1. Make the D partition 'Primary'. You can do that with Disk Management or with the bootable CD of Partition Wizard
(last entry on the page).
Burn that to CD and boot with that CD. Highlight the D partition, go to the Partition tab > Modify.
2. Deactivate the C partition and then activate the D partition. Do that with the Partition Wizard CD.
3. Run Startup/repair 3 times to get the bootmgr on D. Startup Repair - Run 3 Separate Times
4. Test whether your system boots - if yes, you can delete the old C.