I think what may have happened, is when you installed XP, it defaulted to formatting your "D" drive as NTFS.
ME cannot read files formatted this way, which is why you cannot see "D" from "C"!
However, the reverse is not true, and XP can read files formatted as FAT32, the likely format of your "C" drive.
I don't know of a way to "hide" your "C" drive, other than to use third party software.
I am also not sure exactly what you mean by not wanting others to use your programs. Are you referring generally to files, or to applications, as there is conceivably a way around protecting your files from others when in XP.
If you establish a "User Account" and place all the files you do not want others to access within that account, you can then password protect it, and it will become difficult for others to access those files. (Nothing of course is impossible!) But to all but the most determined with a lot of time on their hands, it's a pretty safe option. Security is considerably better in XP, than it was in Win9x.