With Win 7, there are two partitions, a small hidden partition with a few system files, of around 100Mb in size, and the main partition carrying the bulk of the operating system and normally the C: drive.
At first glance, it could be that this hidden partition is not hidden and is getting assigned a drive letter of C: leaving the rest of the drive to get the D: assignment.
Are you imaging using the "disk" option or the "partition" option ? Are you preparing the hard disk in any way prior to deploying the image?
A bit more detail about your process would therefore be helpful to us to better understand exactly what steps you are taking.