I'm not sure If I am up the right tree here, but you can manage partitions in winxp by going
Start > Right clicking 'My Computer' > Clicking Manage > and then under the sub-heading 'Storage' > Select 'Disk Management'.
It is fairly simple to use. Just right click the partition you do not want (In your case D) and select delete.
I'm not sure if you can change the size of the partition of C, in your case to accommodate the space free from your deleted drive d.
What you can do, is just re-create drive D again. That way you will still have all the space you used to have, but it is just spread out over two disks.
If you have a hard drive with a reasonable amount of space, the recommended way to partition is to have at least about 10gb in one partition for windows to run on, and then the rest of the space for things like your programs, and documents.
Hope that is useful to you. Let me know If I am up the wrong tree here!