It appears you didn't boot the Windows 7 DVD to install and instead ran it from XP, which locked out the C letter so you don't have Windows 7 on C as it always is when correctly installed from boot. So I would consider reinstalling it correctly from boot while deleting all partitions during a Clean Install Windows 7.
If you want to keep what you have then since D is a Logical partition which cannot have the System boot files written to it, you must first convert it to Logical. Download free Partition Wizard
Boot CD, burn to CD using Windows Image Burner, then boot CD, rightclick D, select Modify>Set to Primary, click OK.
Next rightclick D to Modify>Set to Active, OK.
Then click on HD to highlight it, from Disk tab select Rebuild MBR, OK, Apply all steps.
If Windows 7 doesn't start at reboot, boot into Windows 7 DVD or REpair CD to run Startup Repair - Run up to 3 Separate Times
until Windows 7 starts and holds the System flag.
You now know Windows 7 can boot from its own partition, so you can boot the PW CD, rightclick C to delete, click OK, then rightclick D to Resize, slide left border all the way to left, OK, APply both steps. Partition Wizard Move/Resize Partition Video Help.