"use partion magic to increase space for drive C."
PowerQuest's Partition Magic or Symantec's Norton Partition Magic is ancient and cannot recognize the slightly different NTFS version made in Vista or Windows 7 properly. You can only fix that if you're dual booting XP and Vista or Windows 7, you load the Recovery Console from the XP CD, choose the XP Windows installation, then you run CHKDSK /R for the partition drive letter the Vista or Windows 7 NTFS partition Windows installation is on.
Vista and Windows 7 have the built in ability to shrink, or increase (if there is un-allocated space made available on the same physical hard drive) the size of hard drive partitions.
Or, you can use newer third party "partition manipulation" programs that can recognize the NTFS version Vista and Windows 7 uses properly.
E.g. the freeware Easeus Partition Master Home Edition.
If you DO have a brand name system that has it's original software installation on it, intact, if it has only one physical hard drive, the hard drive has at least one other smaller partition on it other than the large one you see as C in Windows. In that case, the brand name either chooses to not show the other smaller partition as D in Windows, or it does but you are either not allowed to add data or change data on it, or you are strongly discouraged from doing so.
In that case you can't install anything on D, or delete the data on D so you can do that, unless you're not concerned about possibly not being able to install the original contents of the C partition by using the data on the D partition that the brand name placed there.
If you DO NOT have a brand name system that has it's original software installation on it, or if you do but the original data contents of D on the same hard drive as Windows is on have been deleted...
"Your data is being stored on the C: drive because that's where YOU are putting it."
If you have more than one hard drive partition you can install data on, you DO NOT have to install most programs that did not come with Windows on the C partition. In most cases, you can choose something other than the default installation location on the C partition, when you choose something other than the default Express or similar installation, such as Custom, or you can change just the drive letter at the beginning of the default installation location to that of another hard drive partition.In that case, the vast majority of the data is installed on the other partition, and only a little necessary data is installed on the C partition.
You can install things, or copy things to and delete the original file, other than programs that are already installed on C, on / onto any available hard drive partition that you can install data on - e.g. documents, music, pictures, videos, movies.
You can un-install programs already installed on C, then install them again on another hard drive partition, although you may need to back up personal data for those programs if you don't want to lose than data BEFORE you un-install the program.
Where is the logical D drive ?
Do you have only one physical hard drive and D is on the same physical hard drive, or do you have two physical hard drives ?