Copy all the data from drive D to drive C (assuming you only have the two partitions and one hard drive, adjust as needed). Then remove the D partition using FDISK or Partition Magic.
Of course that's only MOVING the data. If you had things INSTALLED to drive D, then you also will need to change all the references for shortcuts and Registry entries. And you must have as much free space on the C partition as the amount of data on the D partition, so that it fits. Using FDISK you will not be able to reclaim the space abandoned by the D partition either, as FDISK can't resize partitions.
The best is to use Partition Magic version 6. It has a MERGE function which will combine the drives, and also comes with DriveMapper, which would probably run after the merge automatically, but can be run separately after the merge. DriveMapper will search out all references to a drive letter that you specify (in this case D: ) and replace them with the new drive, C:, in the Registry and all shortcuts AND any .ini files and such.