If you're using Vista, Disk Management has the facility to "shrink" or "extend" a volume. This means that you could "shrink" D Drive to create Unallocated Space and then "extend" C Drive to take in the Unallocated Space.
THE PROBLEM others have encountered when doing this is that the now smaller D Drive is between C Drive and the Unallocated Space preventing C Drive from using it.
Have a look at the free "Easeus Partition Master" software which should let you manipulate your partition structure as you want it click here
The problem is when I installed Windows 7 over the vista program (clean install), it stuffed all the old files somewhere, and this is what's taking up the space. I want get rid of these but I can't even find them. So if anyone can tell me how this is done I will be eternally grateful.