*** WARNING ***
These instructions can destroy data or leave you unable to boot if not followed carefully - read full post 1st
*** WARNING ***
Since you're using XP, make use of the Disk Management snap-in. This is found by right-clicking My Computer, choosing Manage, and clicking on Disk Management. The colors of your drives (read partitions) will tell you if this can be done. Dark Blue = Primary Partition; Bright Blue = Logical drive in an extended partition; Green Border = Extended partition.
The graphic representation of the physical drive goes left to right, low to high.
Use this to tell you if what you want to do can be done without destroying data. Since your description of what you want to do is to extend your D drive/partition to take part or all of C, this will not be possible without a more involved process.
You cannot extend a system or boot partition/drive - these will show as (System) or (Boot). You can extend a drive or partition into free space to its immediate right - into contiguous higher blocks of the same partition type. You cannot extend a drive/partition into space to its left - into contiguous lower blocks.
You can create free space from an existing partition by formatting a drive/partition and then right-click on it, choose Change drive letter or paths, and click Remove. Keep in mind DMMMC can only deal with whole partitions or logical drives, it cannot change partition size by arbitrary amounts.
If you need to expand your D partition into lower blocks, you cant get there from here. The easiest answer would be to back up your data and installation programs, repartition your drive & reinstall windows.
If you simply need more space, consider formatting C and offloading some data to it - would still be my recommendation to backup & reinstall.