If C: was your "Master" drive when you installed XP, it will have written essential boot files to the first sector of C: (just a few bytes), but it's enough to stop XP from booting.
The real answer is, try it and see. Just take the cover off the machine and unplug the power and data cables to C: and see if it boots. If you get a message similar to "cannot find drive" (can't remember the exact message), you will know you can't remove the drive.
However, you should be able to make your existing D: drive bootable by using the "fixboot" command in the recovery console click here