If you know for a fact that it's booting to C: instead of D:, and you know for a fact that you have an OS installed on D:, then you definately have two OS's.
In that case, find the boot.ini file and edit it to make your D: drive OS the booting, sole OS.
You can also set your D: drive to be the primary boot drive in your BIOS, which should force the system to detect the OS on D: and boot right from that.
I'm curious why you installed an OS on D:. Are you running two different OS's?